Steam-turbine power plant and steam turbine

ABSTRACT

A steam turbine in which principal components to be exposed to high temperatures are all made of ferritic steel, whereby the temperatures of main steam and reheat steam can be increased to 610-660 (°C.). The rotor shaft (23 in FIG. 1) of the steam turbine is made of ferritic forged steel whose 100,000-hour creep rupture strength is at least 15 (kg/mm 2 ) at the service temperature of the rotor shaft. Likewise, the casing (18) is made of ferritic cast steel whose 100,000-hour creep rupture strength is at least 10 (kg/mm 2 ). The steam turbine of high thermal efficiency can be applied to a steam-turbine power plant.

This is a continuation of application Ser. No. 08/391,945, filed Feb.21, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel steam turbine of highefficiency and high temperature, and more particularly to a steamturbine in which a main steam temperature and/or a reheat steamtemperature are/is 620 (°C.) or above. It also relates to asteam-turbine power plant which employs such steam turbines.

2. Description of the Related Art

Conventional steam turbines have had a steam temperature of 566 (°C.) atmaximum and a steam pressure of 246 (atg).

It is desired, however, to heighten the efficiencies of thermal powerplants from the viewpoints of the exhaustion of fossil fuel such aspetroleum and coal, the saving of energy, and the prevention ofenvironmental pollution. For enhancing the power generationefficiencies, it is the most effective expedient to raise the steamtemperatures of the steam turbines. Regarding materials for suchhigh-efficiency turbines, 1Cr--1Mo--1/4V ferritic low-alloy forged steeland 11Cr--1Mo--V--Nb--N forged steel are known as rotor materials, while1Cr--1Mo--1/4V ferritic low-alloy cast steel and 11Cr--1Mo--V--Nb--Ncast steel are known as casing materials. Among these materials,austenitic alloys disclosed in the official gazette of Japanese PatentApplications Laid-open No. 180044/1987 and No. 23749/1986, andmartensitic steel disclosed in the official gazette of Japanese PatentApplications, Laid-open No. 147948/1992, No. 290950/1990 and No.371551/1992 are especially known as materials whose high-temperaturestrengths are superior.

Although, in the laid-open applications mentioned above, the rotormaterials, the casing materials, etc. are disclosed, almost noconsideration is given to the steam turbines and the thermal powerplants which are accompanied by the higher steam temperatures as statedabove.

Further, a supercritical steam turbine is known from the officialgazette of Japanese Patent Applications, Laid-open No. 248806/1987, buta plant system as a whole is not considered at all.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a steam turbine whichpermits a heightened steam temperature of 610-660 (°C.) byheat-resisting ferritic steel and which exhibits a high thermalefficiency, and a steam-turbine power plant which employs the steamturbine.

Another object of the present invention is to provide steam turbineswhose running temperatures are 610-660 (°C.) and whose basic designs aresubstantially the same, and a steam-turbine power plant which employsthe steam turbines.

The present invention consists in improvement to a steam-turbine powerplant having a high-pressure turbine and an intermediate-pressureturbine which are joined to each other, and low-pressure turbines whichare connected in tandem. The improvement comprises that steam inlet ofeach of the high-pressure and intermediate-pressure turbines which leadsto moving blades of a first-stage included in each of the high-pressureand intermediate-pressure turbines being at a temperature of 610-660(°C.) (preferably 615-640 (°C.), and more preferably 620-630 (°C.)).Further, that steam inlet of each of the low-pressure turbines whichleads to moving blades of a first stage included in each of thelow-pressure turbines is at a temperature of 380-475 (°C.) (preferably400-430 (°C.)), and a rotor shaft, the moving blades, fixed blades and acasing, which are included in each of the high-pressure andintermediate-pressure turbines and which are exposed to the temperatureof the steam inlet of each of the high-pressure andintermediate-pressure turbines, are made of high-strength martensiticsteel which contains 8-13 (weight-%) of Cr.

Further, the present invention consists in an improvement to asteam-turbine power plant having a high-pressure turbine and anintermediate-pressure turbine which are joined to each other, andlow-pressure turbines which are connected in tandem. In the improvement,that steam inlet of each of the high-pressure and intermediate-pressureturbines which leads to moving blades of a first stage included in eachof the high-pressure and intermediate-pressure turbines is at atemperature of 610-660 (°C.) (preferably 615-640 (°C.), and morepreferably 620-630 (°C.)). Further, that steam inlet of each of thelow-pressure turbines which leads to moving blades of a first stageincluded in each of the low-pressure turbines is at a temperature of380-475 (°C.) (preferably 400-430 (°C.)), and a rotor shaft, fixedblades and a casing, which are included in each of the high-pressure andintermediate-pressure turbines and which are exposed to the temperatureof the steam inlet of each of the high-pressure andintermediate-pressure turbines, are made of high-strength martensiticsteel which contains 8-13 (weight-%) of Cr.

Further, the present invention consists in a steam turbine comprising arotor shaft, moving blades which are assembled on the rotor shaft, fixedblades which guide inflow of steam to the moving blades, and an innercasing which holds the fixed blades. The steam flows into a first stageof the moving blades at a temperature of 610-660 (°C.) and under apressure of at least 250 (kg/cm²) (preferably 246-316 (kg/cm²)) or170-200 (kg/cm²). The rotor shaft and, at least, first-stage ones of themoving blades and the fixed blades are made of high-strength martensiticsteel of fully-tempered martensitic structure which exhibits a 10⁵ -hourcreep rupture strength of at least 15 (kg/mm²) (preferably 17 (kg/mm²))at a temperature corresponding to the respective steam temperatures(preferably 610 (°C.), 625 (°C.), 640 (°C.), 650 (°C.) and 660 (°C.)),and which contains 9.5-13 (weight-%) (preferably 10.5-11.5 (weight-%))of Cr, and the inner casing is made of martensitic cast steel whichexhibits a 10⁵ -hour creep rupture strength of at least 10 (kg/mm²)(preferably 10.5 (kg/mm²)) at the temperature corresponding to therespective steam temperatures, and which contains 8-9.5 (weight-%) ofCr.

Further, the present invention consists in an improvement to a steamturbine having a rotor shaft, moving blades which are assembled on therotor shaft, fixed blades which guide inflow of steam to the movingblades, and an inner casing which holds the fixed blades. In theimprovement, the rotor shaft and, at least, first-stage ones of themoving blades and the fixed blades are made of high-strength martensiticsteel which contains 0.05-0.20 (%) of C, at most 0.15 (%) of Si,0.03-1.5 (%) of Mn, 9.5-13 (%) of Cr, 0.05-1.0 (%) of Ni, 0.05-0.35 (%)of V, 0.01-0.20 (%) of Nb, 0.01-0.06 (%) of N, 0.05-0.5 (%) of Mo,1.0-4.0 (%) of W, 2-10 (%) of Co and 0.0005--0.03 (%) of B, and whichhas at least 78 (%) of Fe, the percentages being given in terms ofweight, and the inner casing is made of high-strength martensitic steelwhich contains 0.06-0.16 (%) of C, at most 0.5 (%) of Si, at most 1 (%)of Mn, 0.2-1.0 (%) of Ni, 8-12 (%) of Cr, 0.05-0.35 (%) of V, 0.01-0.15(%) of Nb, 0.01-0.8 (%) of N, at most 1 (%) of Mo, 1-4 (%) of W and0.0005-0.03 (%) of B, and which has at least 85 (%) of Fe, thepercentages being given in terms of weight.

Further, the present invention consists in the improvement to ahigh-pressure steam turbine having a rotor shaft, moving blades whichare assembled on the rotor shaft, fixed blades which guide inflow ofsteam to the moving blades, and an inner casing which holds the fixedblades, wherein the moving blades are arranged including at least 10stages on each side in a lengthwise direction of the rotor shaft, excepta first stage which is of double flow, and the rotor shaft has adistance (L) of at least 5000 (mm) (preferably 5200-5500 (mm)) betweencenters of bearings in which it is journaled, and a minimum diameter (D)of at least 600 (mm) (preferably 620-700 (mm)) at its parts whichcorrespond to the fixed blades, a ratio (L/D) between the distance (L)and the diameter (D) being 8.0-9.0 (preferably 8.3-8.7), and it is madeof high-strength martensitic steel which contains 9-13 (weight-%) of Cr.

Further, the present invention consists in the improvement in anintermediate-pressure steam turbine having a rotor shaft, moving bladeswhich are assembled on the rotor shaft, fixed blades which guide inflowof steam to the moving blades, and an inner casing which holds the fixedblades, wherein the moving blades have a double-flow construction inwhich at least 6 stages are included on each side in a lengthwisedirection of the rotor shaft, in a bilaterally symmetric arrangement onboth sides, and in which the first stages of the arrangement areassembled on a central part of the rotor shaft in the lengthwisedirection, and the rotor shaft has a distance (L) of at least 5200 (mm)(preferably 5300-5800 (mm)) between centers of bearings in which it isjournaled, and a minimum diameter (D) of at least 620 (mm) (preferably620-680 (mm)) at its parts which correspond to the fixed blades, a ratio(L/D) between the distance (L) and the diameter (D) being 8.2-9.2(preferably 8.5-9.0), and it is made of high-strength martensitic steelwhich contains 9-13 (weight-%) of Cr.

Further, the present invention consists in the improvement to alow-pressure steam turbine having a rotor shaft, moving blades which areassembled on the rotor shaft, fixed blades which guide inflow of steamto the moving blades, and an inner casing which holds the fixed blades,wherein the moving blades have a double-flow construction in which atleast 8 stages are included on each side in a lengthwise direction ofthe rotor shaft, in a bilaterally symmetric arrangement on both sides,and in which the first stages of the arrangement are assembled on acentral part of the rotor shaft in the lengthwise direction, the rotorshaft has a distance (L) of at least 7200 (mm) (preferably 7400-7600(mm)) between centers of bearings in which it is journaled, and aminimum diameter (D) of at least 1150 (mm) (preferably 1200-1350 (mm))at its parts which correspond to the fixed blades, a ratio (L/D) betweenthe distance (L) and the diameter (D) being 5.4-6.3 (preferably5.7-6.1), and it is made of Ni--Cr--Mo--V low-alloy steel which contains3.25-4.25 (weight-%) of Ni, and each of the final-stage moving blades ofthe arrangement has a length of at least 40 (inches) and is made of aTi-based alloy.

Further, the present invention consists in the improvement to asteam-turbine power plant having a high-pressure turbine and anintermediate-pressure turbine which are joined to each other, and twolow-pressure turbines which are connected in tandem, wherein that steaminlet of each of the high-pressure and intermediate-pressure turbineswhich leads to moving blades of a first stage included in each of thehigh-pressure and intermediate-pressure turbines is at a temperature of610-660 (°C.), that steam inlet of the low-pressure turbine which leadsto moving blades of a first stage included in the low-pressure turbineis at a temperature of 380-475 (°C.), the first-stage moving blade ofthe high-pressure turbine, and that part of a rotor shaft of thehigh-pressure turbine on which the first-stage moving blade is assembledare held at metal temperatures which are not, at least, 40 (°C.) lowerthan the temperature of the steam inlet of the high-pressure turbineleading to the first-stage moving blade (preferably, the metaltemperatures are 20-35 (°C.) lower than the steam temperature), thefirst-stage moving blade of the intermediate-pressure turbine, and thatpart of a rotor shaft of the intermediate-pressure turbine on which thefirst-stage moving blades are assembled are held at metal temperatureswhich are not, at least, 75 (°C.) lower than the temperature of thesteam inlet of the intermediate-pressure turbine leading to thefirst-stage moving blade (preferably, the metal temperatures are 50-70(°C.) lower than the steam temperature), and the rotor shaft of each ofthe high-pressure and intermediate-pressure turbines and, at least, thefirst-stage one of the moving blades of each of the high-pressure andintermediate-pressure turbines are made of martensitic steel whichcontains 9.5-13 (weight-%) of Cr.

Further, the present invention consists in the improvement to acoal-fired power plant having a coal-fired boiler, steam turbines whichare driven by steam developed by the boiler, and one or more, preferablytwo, generators which are driven by the steam turbines and which cangenerate an output of at least 1000 (MW), wherein the steam turbinesinclude a high-pressure turbine, an intermediate-pressure turbine whichis joined to the high-pressure turbine, and two low-pressure turbines,that steam inlet of each of the high-pressure and intermediate-pressureturbines which leads to moving blades of a first stage included in eachof the high-pressure and intermediate-pressure turbines is at atemperature of 610-660 (°C.), that steam inlet of the low-pressureturbine which leads to moving blades of a first stage included in thelow-pressure turbine is at a temperature of 380-475 (°C.), the steamheated by a superheater of the boiler to a temperature which is at least3 (°C.) (preferably 3-10 (°C.), more preferably 3-7 (°C.)) higher thanthe temperature of the steam inlet of the high-pressure turbine leadingto the first-stage moving blade thereof is caused to flow into thefirst-stage moving blade of the high-pressure turbine, the steam havingcome out of the high-pressure turbine is heated by a reheater of theboiler to a temperature which is at least 2 (°C.) (preferably 2-10(°C.), more preferably 2-5 (°C.)) higher than the temperature of thesteam inlet of the intermediate-pressure turbine leading to thefirst-stage moving blade thereof, whereupon the heated steam is causedto flow into the first-stage moving blade of the intermediate-pressureturbine, and the steam having come out of the intermediate-pressureturbine is heated by an economizer of the boiler to a temperature whichis at least 3 (°C.) (preferably 3-10 (°C.), more preferably 3-6 (°C.))higher than the temperature of the steam inlet of the low-pressureturbine leading to the first-stage moving blade thereof, whereupon theheated steam is caused to flow into the first-stage moving blade of thelow-pressure turbine.

Further, the present invention consists in the improvement to thelow-pressure steam turbine stated before; wherein that steam inlet ofthe low-pressure turbine which leads to a first-stage one of the movingblades is at a temperature of 380-475 (°C.) (preferably 400-450 (°C.)),and the rotor shaft is made of low-alloy steel which contains 0.2-0.3(%) of C, at most 0.05 (%) of Si, at most 0.1 (%) of Mn, 3.25-4.25 (%)of Ni, 1.25-2.25 (%) of Cr, 0.07-0.20 (%) of Mo, 0.07-0.2 (%) of V andat least 92.5 (%) of Fe, the percentages being given in terms of weight.

The present invention consists in the improvement to the high-pressuresteam turbine stated before; wherein the moving blades are arrangedincluding at least 7 stages (preferably 9-12 stages), and they havelengths of 35-210 (mm) in a region from an upstream side of the steamflow to a downstream side thereof, diameters of those parts of the rotorshaft on which the moving blades are assembled are larger than diametersof those parts of the rotor shaft which correspond to the fixed blades;and widths of the moving-blade assembling parts of the rotor shaft in anaxial direction of the rotor shaft being stepwise larger on thedownstream side than on the upstream side at, at least, 3 stages(preferably 4-7 stages), and their ratios to the lengths of the movingblades decrease from the upstream side toward the downstream side withina range of 0.6-1.0 (preferably 0.65-0.95).

Further, in the high-pressure steam turbine stated before, the presentinvention consists in the improvement wherein the moving blades arearranged including at least 7 stages, and they have lengths of 35-210(mm) in a region from an upstream side of the steam flow to a downstreamside thereof, ratios between the lengths of the moving blades of therespectively adjacent stages are at most 1.2 (preferably 1.10-1.15), andthey increase gradually toward the downstream side, and the lengths ofthe moving blades are larger on the downstream side than on the upstreamside.

Further, in the high-pressure steam turbine stated before, the presentinvention consists in the improvement wherein the moving blades arearranged including at least 7 stages, and they have lengths of 35-210(mm) in a region from an upstream side of the steam flow to a downstreamside thereof, and widths of those parts of the rotor shaft whichcorrespond to the fixed blades, the widths being taken in an axialdirection of the rotor shaft, are stepwise smaller on the downstreamside than on the upstream side at, at least, 2 stages (preferably 2-4stages), and their ratios to the lengths of the downstream-side movingblades decrease stepwise toward the downstream side within a range of0.65-1.8 (preferably 0.7-1.7).

The present invention consists in the improvement in theintermediate-pressure steam turbine stated before, wherein the movingblades have a double-flow construction in which at least 6 stages(preferably 6-9 stages) are included on each side in a lengthwisedirection of the rotor shaft, in a bilaterally symmetric arrangement onboth sides, and they have lengths of 100-300 (mm) in a region from anupstream side of the steam flow to a downstream side thereof, diametersof those parts of the rotor shaft on which the moving blades beingassembled are larger than diameters of those parts of the rotor shaftwhich correspond to the fixed blades, and widths of the moving-bladeassembling parts of the rotor shaft in an axial direction of the rotorshaft being-stepwise larger on the downstream side than on the upstreamside at, at least, 2 stages (preferably 3-6 stages), and their ratios tothe lengths of the moving blades decrease from the upstream side towardthe downstream side within a range of 0.45-0.75 (preferably 0.5-0.7).

Further, in the intermediate-pressure steam turbine stated before, thepresent invention consists in the improvement wherein the moving bladeshave a double-flow construction in which at least 6 stages are includedon each side in a lengthwise direction of the rotor shaft, in abilaterally symmetric arrangement on both sides, and they have lengthsof 100-300 (mm) in a region from an upstream side of the steam flow to adownstream side thereof, and the lengths of the respectively adjacentmoving blades are larger on the downstream side than on the upstreamside, and their ratios are at most 1.3 (preferably 1.1-1.2) and increasegradually toward the downstream side.

Further, in the intermediate-pressure steam turbine stated before, thepresent invention consists in the improvement wherein the moving bladeshave a double-flow construction in which at least 6 stages are includedon each side in a lengthwise direction of the rotor shaft, in abilaterally symmetric arrangement on both the sides, and they havelengths of 100-300 (mm) in a region from an upstream side of the steamflow to a downstream side thereof, and widths of those parts of therotor shaft which correspond to the fixed blades, the widths being takenin an axial direction of the rotor shaft, are stepwise smaller on thedownstream side than on the upstream side at, at least, 2 stages(preferably 3-6 stages), and their ratios to the lengths of thedownstream-side moving blades decrease stepwise toward the downstreamside within a range of 0.45-1.60 (preferably 0.5-1.5).

The present invention consists in the improvement in the low-pressuresteam turbine stated before; wherein the moving blades have adouble-flow construction in which at least 8 stages (preferably 8-10stages) are included on each side in a lengthwise direction of the rotorshaft, in a bilaterally symmetric arrangement on both sides, and theyhave lengths of 90-1300 (mm) in a region from an upstream side of thesteam flow to a downstream side thereof; diameters of those parts of therotor shaft on which the moving blades are assembled are larger thandiameters of those parts of the rotor shaft which correspond to thefixed blades; and widths of the moving-blade assembling parts of therotor shaft in an axial direction of the rotor shaft are stepwise largeron the downstream side than on the upstream side at, at least, 3 stages(preferably 4-7 stages), and their ratios to the lengths of the movingblades decrease from the upstream side toward the downstream side withina range of 0.15-1.0 (preferably 0.15-0.91).

Further, in the low-pressure steam turbine stated before, the presentinvention consists in the improvement wherein the moving blades have adouble-flow construction in which at least 8 stages are included on eachside in a lengthwise direction of the rotor shaft, in a bilaterallysymmetric arrangement on both sides, and they have lengths of 90-1300(mm) in a region from an upstream side of the steam flow to a downstreamside thereof; and the lengths of the moving blades of the respectivelyadjacent stages are larger on the downstream side than on the upstreamside, and their ratios increase gradually toward the downstream sidewithin a range of 1.2-1.7 (preferably 1.3-1.6).

Further, in the low-pressure steam turbine stated before, the presentinvention consists in the improvement wherein the moving blades have adouble-flow construction in which at least 8 stages are included on eachside in a lengthwise direction of the rotor shaft, in a bilaterallysymmetric arrangement on both sides, and they have lengths of 90-1300(mm) in a region from an upstream side of the steam flow to a downstreamside thereof; and widths of those parts of the rotor shaft whichcorrespond to the fixed blades, the widths being taken in an axialdirection of the rotor shaft, are stepwise larger on the downstream sidethan on the upstream side at, at least, 3 stages (preferably 4-7stages), and their ratios to the lengths of the respectively adjacentmoving blades on the downstream side decrease stepwise toward thedownstream side within a range of 0.2-1.4 (preferably 0.25-1.25).

The present invention consists in the improvement in a high-pressuresteam turbine having a rotor shaft, moving blades which are assembled onthe rotor shaft, fixed blades which guide inflow of steam to the movingblades, and an inner casing which holds the fixed blades; wherein themoving blades are arranged including at least 7 stages; diameters ofthose parts of the rotor shaft which correspond to the fixed blades aresmaller than diameters of those parts of the rotor shaft whichcorrespond to the assembled moving blades; widths of the rotor shaftparts corresponding to the fixed blades, in an axial direction of therotor shaft are stepwise larger on an upstream side of the steam flowthan on a downstream side thereof at, at least, 2 of the stages(preferably 2-4 stages), and the width between the final stage of themoving blades and the stage thereof directly preceding the final stageis 0.75-0.95 (preferably 0.8-0.9, more preferably 0.84-0.88) times aslarge as the width between the second stage and the third stage of themoving blades; and widths of the rotor shaft parts corresponding to theassembled moving blades, in the axial direction of the rotor shaft arestepwise larger on the downstream side of the steam flow than on theupstream side thereof at, at least, 3 of the stages (preferably 4-7stages), and the axial width of the final stage of the moving blades is1-2 (preferably 1.4-1.7) times as large as the axial width of the secondstage of the moving blades.

The present invention consists in the improvement in anintermediate-pressure steam turbine having a rotor shaft, moving bladeswhich are assembled on the rotor shaft, fixed blades which guide inflowof steam to the moving blades, and an inner casing which holds the fixedblades; wherein the moving blades are arranged including at least 6stages; diameters of those parts of the rotor shaft which correspond tothe fixed blades are smaller than diameters of those parts of the rotorshaft which correspond to the assembled moving blades; widths of therotor shaft parts corresponding to the fixed blades, in an axialdirection of the rotor shaft are stepwise larger on an upstream side ofthe steam flow than on a downstream side thereof at, at least, 2 of thestages (preferably 3-6 stages), and the width between the final stage ofthe moving blades and the stage thereof directly preceding the finalstage is 0.55-0.8 (preferably 0.6-0.7) times as large as the widthbetween the first stage and the second stage of the moving blades; andwidths of the rotor shaft parts corresponding to the assembled movingblades, in the axial direction of the rotor shaft are stepwise larger onthe downstream side of the steam flow than on the upstream side thereofat, at least, 2 of the stages (preferably 3-6 stages), and the axialwidth of the final stage of the moving blades is 0.8-2 (preferably1-1.5) times as large as the axial width of the first stage of themoving blades.

The present invention consists in the improvement in a low-pressuresteam turbine having a rotor shaft, moving blades which are assembled onthe rotor shaft, fixed blades which guide inflow of steam to the movingblades, and an inner casing which holds the fixed blades; wherein themoving blades have a double-flow construction in which at least 8 stagesare included on each side in an axial direction of the rotor shaft, in abilaterally symmetric arrangement on both sides; diameters of thoseparts of the rotor shaft which correspond to the fixed blades aresmaller than diameters of those parts of the rotor shaft whichcorrespond to the assembled moving blades; widths of the rotor shaftparts corresponding to the fixed blades, in the axial direction of therotor shaft are stepwise larger on an upstream side of the steam flowthan on a downstream side thereof at, at least, 3 of the stages(preferably 4-7 stages), and the width between the final stage of themoving blades and the stage thereof directly preceding the final stageis 1.5-2.5 (preferably 1.7-2.2) times as large as the width between thefirst stage and the second stage of the moving blades; and widths of therotor shaft parts corresponding to the assembled moving blades, in theaxial direction of the rotor shaft, are stepwise larger on thedownstream side of the steam flow than on the upstream side thereof at,at least, 3 of the stages (preferably 4-7 stages), and the axial widthof the final stage of the moving blades is 2-3 (preferably 2.2-2.7)times as large as the axial width of the first stage of the movingblades.

The designs of the high-pressure, intermediate-pressure and low-pressureturbines described above can be rendered similar for any of the servicesteam temperatures, 610-660 (°C.) of the respective turbines.

In the rotor material of the present invention, alloy contents shouldpreferably be controlled so as to become 4-8 in terms of a Cr equivalentwhich is computed by a formula given below, in order that a superiorhigh-temperature strength, a low-temperature toughness and a highfatigue strength may be attained from the fully-tempered martensiticstructure.

Besides, in the heat-resisting cast steel of the present invention,which is used as the casing material, alloy contents should preferablybe controlled so as to become 4-10 in terms of the Cr equivalent whichis computed by the formula given below, in order that a superiorhigh-temperature strength, a low-temperature toughness and a highfatigue strength may be attained by controlling the alloyingconstituents so as to establish a martensitic structure tempered to atleast 95 (%), in other words, containing at most 5 (%) of δ (delta)ferrite.

    Cr equivalent=Cr+6Si+4Mo+1.5W+11V+5Nb-40C-30N-30B-2Mn-4Ni-2Co

Regarding the 12Cr heat-resisting steel of the present invention,especially in a case where the steel is used with steam at or above 621(°C.), it should preferably be endowed with a 625-°C. 10⁵ -h creeprupture strength of at least 10 (kgf/mm²) and a room-temperatureabsorbed impact energy of at least 1 (kgf-m).

Now, the materials specified in the present invention will be itemizedas (1)-(3) below.

(1) There will be elucidated the reasons for restricting theconstituents of the heat-resisting ferritic steel which is used in thepresent invention for making the rotors, blades, nozzles andinner-casing tightening bolts of the high-pressure andintermediate-pressure steam turbines, and the first-stage diaphragm ofthe intermediate-pressure portion:

The constituent C (carbon) is an element which is indispensable toensuring hardenability upon quenching, and precipitating carbides in atempering heat-treatment process so as to enhance a high-temperaturestrength. Besides, the element C is required at a level of at least 0.05(%) in order to attain a high tensile strength. However, in a case wherethe C content exceeds 0.20 (%), the ferritic steel comes to have anunstable metallographic structure and spoils the long-time creep rupturestrength thereof when exposed to high temperatures for a prolongedperiod of time. Therefore, the C content is restricted to within0.05-0.20 (%). It should desirably be within 0.08-0.13 (%), andparticularly preferably be within 0.09-0.12 (%)

The constituent Mn (manganese) is added as a deoxidizer etc., and thedeoxidizing effect thereof is achieved by a small amount of addition. Alarge amount of addition exceeding 1.5 (%) is unfavorable because itlowers the creep rupture strength. Especially; a range of 0.03-0.20 (%)or a range of 0.3-0.7 (%) is preferable, and a range of 0.35-0.65 (%) ismore preferable for the latter. As the Mn content is made lower, ahigher strength is attained. On the other hand, as the Mn content ismade higher, the workability of the ferritic steel improves.

The constituent Si (silicon) is also added as a deoxidizer, but the Sideoxidation is dispensed if a steelmaking technique such as the vacuum Cdeoxidation or the like is made. A lower Si content is effective toprevent the production of the deleterious δ ferrite structure, and toprevent the degradation of the toughness of the ferritic steelattributed to grain-boundary segregation, etc. Accordingly, the additionof the constituent Si needs to be suppressed to 0.15 (%) or below. TheSi content of the ferritic steel should desirably be at most 0.07 (%),and should particularly preferably be less than 0.04 (%).

The constituent Ni (nickel) is an element which is very effective toheighten the toughness and to prevent the production of the δ ferrite.The addition of the element Ni at a level of less than 0.05 (%) isunfavorable because it has an insufficient effect, and the additionthereof at more than 1.0 (%) is also unfavorable because of degradationin the creep rupture strength. Especially, a range of 0.3-0.7 (%) ispreferable, and a range of 0.4-0.65 (%) is more preferable.

The constituent Cr (chromium) is an element which is indispensable toenhancing the high-temperature strength and high-temperature oxidationresistance of the ferritic steel. The element Cr is required at least 9(%). However, when the Cr content exceeds 13 (%), the deleterious δferrite structure is produced, which lowers the high-temperaturestrength and the toughness. Therefore, the Cr content is restricted towithin 9-12 (%). Especially, a range of 10-12 (%) is preferable, and arange of 10.8-11.8 (%) is more preferable.

The addition of the constituent Mo (molybdenum) is intended to enhancethe high-temperature strength. However, in a case where the constituentW (tungsten) is contained at a level of more than 1 (%), as in the steelof the present invention, Mo addition at a level of exceeding 0.5 (%)lowers the toughness and fatigue strength of the ferritic steel.Therefore, the Mo content is limited to, at most, 0.5 (%). Especially, arange of 0.05-0.45 (%) is preferable, and a range of 0.1-0.2 (%) is morepreferable.

The constituent W (tungsten) suppresses the coarsening of carbides dueto the agglomerations thereof at high temperatures, and it turns thematrix of the ferritic steel into a solid solution and strengthens thismatrix. It is therefore effective to remarkably enhance the long-termstrength of the ferritic steel at the high temperatures of at least 620(°C.). The W content of the ferritic steel should preferably be 1-1.5(%) at 620 (°C.), 1.6-2.0 (%) at 630 (°C.), 2.1-2.5 (%) at 640 (°C.),2.6-3.0 (%) at 650 (°C.) and 3.1-3.5 (%) at 660 (°C.). Besides, when theW content exceeds 3.5 (%), the δ ferrite is produced, which lowers thetoughness. Therefore, the W content is restricted to within 1-3.5 (%).Especially, a range of 2.4-3.0 (%) is preferable, and a range of 2.5-2.7(%) is more preferable.

The constituent V (vanadium) is effective to heighten the creep rupturestrength by precipitating the carbonitrides of this constituent V. Whenthe V content of the ferritic steel is less than 0.05 (%), the effect isinsufficient. On the other hand, when the V content exceeds 0.3 (%), theδ ferrite is produced, which lowers the fatigue strength. Especially, arange of 0.10-0.25 (%) is preferable, and a range of 0.15-0.23 (%) ismore preferable.

The constituent Nb (niobium) is an element which is very effective toprecipitate NbC (niobium carbide) and enhance the high-temperaturestrength. However, when the element Nb is added in an excessively largeamount, the coarse grains of eutectic NbC appear, especially in alarge-sized steel ingot, which causes significant lowering of thestrength and precipitation of the δ ferrite, which lowers the fatiguestrength. It is therefore necessary to suppress the amount of theelement Nb to 0.20 (%) or below. On the other hand, when the Nb amountis less than 0.01 (%), the effect is insufficient. Especially, a rangeof 0.02-0.15 (%) is preferable, and a range of 0.04-0.10 (%) is morepreferable.

The constituent Co (cobalt) is an important element, and is a featurewhich distinguishes the present invention from the prior-art techniques.In the present invention, owing to the addition of the element Co, thehigh-temperature strength is remarkably improved, and the toughness isalso heightened.

These effects are considered to be based on the interaction between theelements Co and W, and they are the characterizing phenomena of thealloy of the present invention containing the element W in the amount ofat least 1 (%). In order to realize such effects of the element Co, thelower limit of the Co amount in the alloy of the present invention isset at 2.0 (%) On the other hand, even when the element Co is added inexcess, greater effects are not attained, and moreover, the ductility ofthe ferritic steel is lowered. Therefore, the upper limit of the Coamount is set at 10 (%). The Co amount should desirably be 2-3 (%) for620 (°C.), 3.5-4.5 (%) for 630 (°C.), 5-6 (%) for 640 (°C.), 6.5-7.5 (%)for 650 (°C.), and 8-9 (%) for 660 (°C.).

The constituent N (nitrogen) is also an important element and is afeature which distinguishes the present invention from the prior-arttechniques. The element N is effective to improve the creep rupturestrength and to prevent the production of the δ ferrite structure.However, when the N content of the ferritic steel is less than 0.01 (%),the effects are not sufficient. On the other hand, when the N contentexceeds 0.05 (%), the toughness is lowered, and the creep rupturestrength is also lowered. Especially, a range of 0.01-0.03 (%) ispreferable, and a range of 0.015-0.025 (%) is more preferable.

The constituent B (boron) is effective to enhance the high-temperaturestrength by the action of intensifying grain boundaries, and the actionof turning into solid solutions in carbides M₂₃ C₆ to hinder the M₂₃ C₆type carbides from coarsening due to the agglomerations thereof. It iseffective to add the constituent B to a level in excess of 0.001 (%).However, when the B content exceeds 0.03 (%), the weldability andforgeability of the ferritic steel are degraded. Therefore, the Bcontent is limited to within 0.001-0.03 (%). It should desirably be0.001-0.01 (%) or 0.01-0.02 (%).

The addition of the constituent/constituents Ta (tantalum), Ti(titanium) or/and Zr (zirconium) is effective to heighten the toughness.A sufficient effect is attained by adding at most 0.15 (%) of Ta, atmost 0.1 (%) of Ti or/and at most 0.1 (%) of Zr singly or incombination. In a case where the constituent Ta is added at a level of0.1 (%) or above, the addition of the constituent Nb (niobium) can beomitted.

The rotor shaft and, at least, the first-stage ones of the moving bladesand fixed blades in the present invention should preferably be made fora steam temperature of 620-630 (°C.) out of steel of fully-temperedmartensitic structure which contains 0.09-0.20 (%) of C, at most 0.15(%) of Si, 0.05-1.0 (%) of Mn, 9.5-12.5 (%) of Cr, 0.1-1.0 (%) of Ni,0.05-0.30 (%) of V, 0.01-0.06 (%) of N, 0.05-0.5 (%) of Mo, 2-3.5 (%) ofW, 2-4.5 (%) of Co, 0.001-0.030 (%) of B, and at least 77 (%) of Fe(iron). Besides, they should preferably be made for a steam temperatureof 635-660 (°C.) out of steel of fully-tempered martensitic structure inwhich the aforementioned Co content is replaced with 5-8 (%), and whichcontains at least 78 (%) of Fe. Especially, a high strength is attainedby decreasing the Mn content to 0.03-0.2 (%) and the B content to0.001-0.01 (%) for both the aforementioned temperatures. The martensiticsteel should particularly preferably contain 0.09-0.20 (%) of C, 0.1-0.7(%) of Mn, 0.1-1.0 (%) of Ni, 0.10-0.30 (%) of V, 0.02-0.05 (%) of N,0.05-0.5 (%) of Mo, and 2-3.5 (%) of W, along with 2-4 (%) of Co and0.001-0.01 (%) of B for a temperature of or below 630 (°C.) or 5.5-9.0(%) of Co and 0.01-0.03 (%) of B for a temperature of 630-660 (°C.).

The Cr equivalent which is obtained by the formula mentioned before isset at 4-10.5 for the rotor shafts of the high-pressure andintermediate-pressure steam turbines, and a range of 6.5-9.5 isparticularly preferable therefor. The same applies to the othercomponents of these steam turbines stated before.

Regarding the rotor material of the high-pressure andintermediate-pressure steam turbines of the present invention, thefatigue strength and the toughness lower due to the coexistence of the δferrite structure. Therefore, the tempered martensitic structure whichis homogeneous, is favorable for the heat-resisting ferritic steel. Inorder to obtain the tempered martensitic structure, the Cr equivalentwhich is computed by the formula mentioned before must be set at, atmost, 10 by controlling the alloy contents. On the other hand, when theCr equivalent is too small, it lowers the creep rupture strength, andhence, it must be set at, at least, 4. Especially, a range of 5-8 ispreferable as the Cr equivalent.

Regarding each of the rotors in the present invention, alloying rawmaterials to be brought into the desired composition are melted in anelectric furnace, the molten materials are deoxidized by carbon vacuumdeoxidation, the deoxidized materials are cast into a metal mold, andthe molded article is forged into an electrode. The electrode thusfabricated is subjected to electroslag remelting, and the resulting slagis forged and formed into the shape of the rotor. The forging must becarried out at a temperature of 1150 (°C.) or below in order to preventforging cracks. After the forged steel has been annealed, it is heatedto 1000-1100 (°C.) and then quenched, and it is tempered twice in thesequence of a temperature range of 550-650 (°C.) and a temperature rangeof 670-770 (°C.). Thus, the steam turbine rotor which is usable in steamat or above 620 (°C.) can be manufactured.

Regarding each of the components in the present invention, whichincludes the blades, nozzles and inner-casing tightening bolts of thehigh-pressure and intermediate-pressure steam turbines, and thefirst-stage diaphragm of the intermediate-pressure portion, an ingot isprepared in such a way that the alloying raw materials to be broughtinto the desired composition are melted by vacuum melting, and that themolten materials are cast in a metal mold in vacuum. The ingot ishot-forged into a predetermined shape at the same temperature as statedbefore. After the forged ingot has been heated to 1050-1150 (°C.), it issubjected to water cooling or oil quenching. Subsequently, the resultingingot is tempered in a temperature range of 700-800 (°C.), and it ismachined into the component of desired shape. The vacuum melting iscarried out under a vacuum condition of 10⁻¹ -10⁻⁴ (mmHg). Inparticular, although the heat-resisting steel in the present inventioncan be applied to all the stages of the blades and nozzles of thehigh-pressure portion and intermediate-pressure portion, they areespecially necessary for the first stages of both the sorts ofcomponent.

The steam-turbine rotor shaft made of the 12 weight-% Cr typemartensitic steel in the present invention should preferably be soconstructed that buildup welding layers of good bearing characteristicsare formed on the surface of the parent metal forming each journalportion of the rotor shaft. More specifically, the buildup weldinglayers are formed in the number of 5-10 by the use of a weld metal beingsteel. The Cr content of the steel as the weld metal is loweredsuccessively from the first layer to any of the second-fourth layers,whereas the layers of and behind the fourth layer are formed of thesteel having an identical Cr content. Herein, the Cr content of the weldmetal for the deposition of the first layer is rendered about 2-6(weight-%) less than that of the parent metal, and the Cr contents ofthe welding layers of and behind the fourth layer are set at 0.5-3(weight-%), preferably at 1-2.5 (weight-%).

In the present invention, the buildup welding is favorable for theimprovement of the bearing characteristics of the journal portion inview of the highest safety, but it becomes very difficult due toincrease in the B content of the steel. Therefore, in the case where theB content is set at 0.02 (%) or above in order to attain a higherstrength, it is recommended to adopt a construction in which the journalportion is inserted into a sleeve made of low-alloy steel having a Crcontent of 1-3 (%), through shrinkage fit. The material composition ofthe sleeve is the same as that of the buildup welding layers to beexplained later.

The buildup welding layers according to the method of the presentinvention need to be in the number of 5-10. Abrupt decrease in theamount of Cr in the first welding layer causes the development of highresidual tensile stress or welding cracks, so that the Cr content of theweld metal of the first welding layer cannot be sharply lowered. Asstated before, therefore, the Cr contents need to be gradually loweredwith the enlarged number of welding layers. Further, since the desiredCr content and a desired thickness need to be held as the surface layerof the journal portion, the welding layers need to be in the number of 5or more. By the way, even when the number of welding layers is largerthan 10, no greater effect is achieved. Regarding a large-sizedstructural member such as the steam-turbine rotor shaft, the buildupwelding layers must not have their composition influenced by the parentmetal and need to be endowed with the desired composition as well as thedesired thickness. Herein, three layers are required as a thickness forpreventing the influence of the parent metal. Besides, layers of desiredcharacteristics need to be stacked on the three layers to a desiredthickness, and at least two layers are required as the desiredthickness. By way of example, a thickness of about 18 (mm) is requiredas the desired thickness of the finally finished buildup welding layers.In order to form such a thickness, at least five buildup welding layersare necessitated even when a final finish margin to be machined isexcluded. The third layer et seq. should preferably be mainly made ofthe tempered martensitic structure from which the carbides have beenprecipitated. Especially, the composition of the fourth welding layer etseq. should preferably contain in terms of weight, 0.01-0.1 (%) of C,0.3-1 (%) of Si, 0.3-1.5 (%) of Mn, 0.5-3 (%) of Cr and 0.1-1.5 (%) ofMo, the balance being Fe.

Moreover, in the buildup welding layers, the Cr content is loweredsuccessively from the first layer to any of the second-fourth layers. Inperforming the buildup welding, welding rods whose Cr contents aregradually lowered are used for the respective layers. Then, the buildupwelding layers of the desired composition can be formed withoutincurring the problem of lowered ductility or welding cracks of thefirst-layer welding zone attributed to the sharp decrease of thechromium content in the first-layer welding zone. In this way, thepresent invention can form the buildup welding layers in which thechromium contents in the vicinities of the parent metal and thefirst-layer zone do not exhibit a very large difference, and in whichthe final layer has the good bearing characteristics as stated above.

The weld metal which is applied to the first-layer welding has itschromium content rendered about 2-6 (weight-%) lower than the chromiumcontent of the parent metal. When the Cr content of the weld metal isless than 2 (%) that of the parent metal, the pertinent Cr content ofthe buildup welding layer cannot be lowered sufficiently, and the effectis slight. To the contrary, when the value exceeds 6 (%), the Cr contentof the buildup welding layer lowers suddenly from that of the parentmetal, and the difference between the Cr contents gives rise to a largedifference between the coefficients of thermal expansion of both themetals, to thereby cause development of high residual tensile stress orwelding cracks. Incidentally, since a higher Cr content results in asmaller coefficient of thermal expansion, the buildup welding layer oflower Cr content has larger coefficient of thermal expansion than theparent metal and is formed with the high residual tensile stress by thewelding. Therefore, the welding with steel of still lower Cr contentproduces a hard layer due to the high residual stress and causesdevelopment of welding cracks. Accordingly, the Cr content of the weldmetal needs to be set at, at most, 6 (%) smaller than that of the parentmetal. Owing to the use of such a weld metal, the chromium content ofthe first-layer welding layer becomes lower than that of the parentmetal by as little as about 1-3 (%) because the weld metal mixes withthe parent metal. Thus, favorable welding is attained.

In the method of the present invention, the layers of and behind thefourth layer need to be formed using weld metal which is made of steelhaving an identical Cr content. In the buildup welding, the buildupwelding layers up to the third layer are influenced by the compositionof the parent metal. Since, however, the fourth buildup welding layer etseq. are composed only of the employed weld metal without thisinfluence, they can be formed to satisfy the characteristics requiredfor the journal portion of the steam-turbine rotor shaft. Besides, asstated before, the thickness of the buildup welding layers required forthe large-sized structural member operating as the steam-turbine rotorshaft is about 18 (mm). Accordingly, in order to ensure the alloyingconstituents required for the final layer and the sufficient thicknessrequired in the case of the constituents, two or more layers aredeposited as the fourth layer et seq. by the use of the weld metalhaving the same Cr content. Thus, the final layer which satisfies thecharacteristics required for the journal portion as stated before can beformed having the sufficient thickness.

(2) There will be elucidated the reasons for restricting theconstituents of the heat-resisting ferritic steel which is used in thepresent invention for making the inner casings, control-valve valvecasings, combinational-reheater-valve valve casings, main-steam leadingpipes, main-steam inlet pipes and reheat-steam inlet pipes of thehigh-pressure and intermediate-pressure steam turbines, the nozzle boxof the high-pressure turbine, the first-stage diaphragm of theintermediate-pressure turbine, and the main-steam inlet flange and elbowand the main-steam stop valve of the high-pressure turbine:

In the casing material of the heat-resisting ferritic cast steel,especially, the Ni/W ratio is controlled to 0.25-0.75, thereby obtainingthe casing material of the heat-resisting cast steel which meets a625-°C. 10⁵ -h creep rupture strength of at least 9 (kgf/mm²) and aroom-temperature absorbed impact energy of at least 1 (kgf-m) that arerequired of the high-pressure and intermediate-pressure inner casings,main-steam stop valve and control valve casing of the turbine under theultra-supercritical pressure of at least 250 (kgf/cm²) at 621 (°C.).

In the heat-resisting cast steel of the present invention used as thecasing material, the Cr equivalent which is computed in terms of thealloy contents (weight-%) of the following formula should preferably becontrolled so as to become 4-10, in order to attain a superiorhigh-temperature strength, a superior low-temperature toughness and ahigh fatigue strength:

    Cr equivalent=Cr+6Si+4Mo+1.5W+11V+5Nb-40C-30N-30B-2Mn-4Ni-2Co

Since the 12Cr heat-resisting steel of the present invention is used inthe steam at or above 621 (°C.), it must be endowed with the 625-°C. 10⁵-h creep rupture strength of at least 9 (kgf/mm²) and theroom-temperature absorbed impact energy of at least 1 (kgf-m). Further,in order to ensure a still higher reliability, this steel shouldpreferably be endowed with a 625-°C. 10⁵ -h creep rupture strength of atleast 10 (kgf/mm²) and a room-temperature absorbed impact energy of atleast 2 (kgf-m).

The constituent C (carbon) is an element which is required at a level ofleast 0.06 (%) in order to attain a high tensile strength. However, in acase where the C content exceeds 0.16 (%), the steel comes to have anunstable metallographic structure and degraded the long-time creeprupture strength thereof when exposed to high temperatures for a longtime period. Therefore, the C content is restricted to within 0.06-0.16(%). It should preferably be within 0.09-0.14 (%).

The constituent N (nitrogen) is effective to improve the creep rupturestrength and to prevent the production of the δ ferrite structure.However, when the N content of the steel is less than 0.01 (%), theeffects are not sufficient. On the other hand, even when the N contentexceeds 0.1 (%), no remarkable effects are attained. Moreover, thetoughness is lowered, and the creep rupture strength is also lowered.Especially, a range of 0.02-0.1 (%) is preferable.

The constituent Mn (manganese) is added for a deoxidizer, and the effectthereof is achieved by a small amount of addition. A large amount ofaddition exceeding 1 (%) is unfavorable because it lowers the creeprupture strength. Especially, a range of 0.4-0.7 (%) is preferable.

The constituent Si (silicon) is also added as a deoxidizer, but the Sideoxidation is dispensed with when the steelmaking technique employed isvacuum C deoxidation or the like. A lower Si content is effective toprevent the production of the deleterious δ ferrite structure.Accordingly, the addition of the constituent Si needs to be suppressedto 0.5 (%) or below. The Si content of the steel should preferably be0.1-0.4 (%).

The constituent V (vanadium) is effective to heighten the creep rupturestrength. When the V content of the steel is less than 0.05 (%), theeffect is insufficient. On the other hand, when the V content exceeds0.35 (%), the δ ferrite is produced which lowers the fatigue strength.Especially, a range of 0.15-0.25 (%) is preferable.

The constituent Nb (niobium) is an element which is very effective toenhance the high-temperature strength. However, when the element Nb isadded in an excessively large amount, the coarse grains of eutectic NbC(niobium carbide) appear especially in a large-sized steel ingot, tothereby cause substantial lowering of the strength and precipitation ofthe δ ferrite which lowers the fatigue strength. It is thereforenecessary to suppress the amount of the element Nb to 0.15 (%) or below.On the other hand, when the Nb amount is less than 0.01 (%), the effectis insufficient. Especially in the case of the large-sized steel ingot,a range of 0.02-0.1 (%) is preferable, and a range of 0.04-0.08 (%) ismore preferable.

The constituent Ni (nickel) is an element which is very effective toheighten the toughness and to prevent the production of the δ ferrite.The addition of the element Ni at a level of less than 0.2 (%) isunfavorable because of insufficient effects, and the addition thereof atthe level of more than 1.0 (%) is also unfavorable because ofdegradation in the creep rupture strength. Especially, a range of0.4-0.8 (%) is preferable.

The constituent Cr (chromium) is effective to improve thehigh-temperature strength and high-temperature oxidation resistance ofthe 12Cr steel. Herein, a Cr content exceeding 12 (%) causes productionof the deleterious δ ferrite structure, and a Cr content below 8 (%)results in an insufficient oxidation resistance to the high-temperaturehigh-pressure steam. Besides, the addition of the element Cr iseffective to enhance the creep rupture strength. However, the Craddition in an excessive amount causes production of the deleterious δferrite structure and for lowering of the toughness. Especially, a rangeof 8.0-10 (%) is preferable, and a range of 8.5-9.5 (%) is morepreferable.

The constituent W (tungsten) is effective to remarkably enhance thehigh-temperature long-term strength of the 12Cr steel. When the amountof the element W is smaller than 1 (%), the effect is insufficient asthe heat-resisting steel which is used at 620-620 (°C.). On the otherhand, when the amount of the element W exceeds 4 (%), the toughness islowered. The W content of the steel should preferably be 1.0-1.5 (%) at620 (°C.), 1.6-2.0 (%) at 630 (°C.), 2.1-2.5 (%) at 640 (°C.), 2.6-3.0(%) at 650 (°C.) and 3.1-3.5 (%) at 660 (°C.).

The constituents W and Ni correlate with each other. The 12Cr steelwhose strength and toughness are both superior, can be obtained bysetting the Ni/W ratio at 0.25-0.75.

The addition of the constituent Mo (molybdenum) is intended to enhancethe high-temperature strength. However, in a case where the constituentW (tungsten) is contained at a level of more than 1 (%) as in the caststeel of the present invention, the Mo addition exceeding 1.5 (%) lowersthe toughness and fatigue strength of the steel. Therefore, the Mocontent is recommended to be at most 1.5 (%). Especially, a range of0.4-0.8 (%) is preferable, and a range of 0.55-0.70 (%) is morepreferable.

The addition of the constituent/constituents Ta (tantalum), Ti(titanium) or/and Zr (zirconium) is effective to heighten the toughness.A sufficient effect is attained by adding at most 0.15 (%) of Ta, atmost 0.1 (%) of Ti or/and at most 0.1 (%) of Zr singly or incombination. In a case where the constituent Ta is added at a level of0.1 (%) or above, the addition of the constituent Nb (niobium) can beomitted.

Regarding the heat-resisting cast steel of the present invention whichis used as the casing material, the fatigue strength and the toughnessare lowered due to the coexistence of the δ ferrite structure.Therefore, the tempered martensitic structure which is homogeneous isfavorable. In order to obtain the tempered martensitic structure, the Crequivalent which is computed by the formula mentioned before must be setat, at most, 10 by controlling the alloy contents. On the other hand,when the Cr equivalent is too small, it lowers the creep rupturestrength, and hence, it must be set at 4 or above. Especially, a rangeof 6-9 is preferable as the Cr equivalent.

The addition of the constituent B (boron) remarkably enhances thehigh-temperature (620 (°C.) or above) creep rupture strength of thesteel. Herein, when the B content of the steel exceeds 0.003 (%), theweldability thereof worsens. Therefore, the upper limit of the B contentis set at 0.003 (%). Especially, the upper limit of the B content of thelarge-sized casing should preferably be set at 0.0028 (%). Further, arange of 0.0005-0.0025 (%) is preferable, and a range of 0.001-0.002 (%)is particularly preferable.

Since the casing covers the high-pressure steam at temperatures of atleast 620 (°C.), it undergoes a high stress ascribable to the internalpressure thereof. From the viewpoint of preventing the creep rupture ofthe casing, therefore, the 10⁵ -h creep rupture strength of at least 10(kgf/mm²) is required of the steel. Moreover, during the startingoperation of the turbine, the casing undergoes a thermal stress at thetime of a low metal temperature. From the viewpoint of preventing thebrittle fracture of the casing, therefore, the room-temperature absorbedimpact energy of at least 1 (kgf-m) is required of the steel. For thehigher temperature side of the steam, the steel can be strengthened bycontaining at most 10 (%) of Co (cobalt). Especially, the Co contentshould preferably be 1-2 (%) for 620 (°C.), 2.5-3.5 (%) for 630 (°C.),4-5 (%) for 640 (°C.), 5.5-6.5 (%) for 650 (°C.), and 7-8 (%) for 660(°C.).

In fabricating the casing having few defects, a high degree ofmanufacturing technology is required because the casing is a large-sizedstructural member whose ingot has a weight of about 50 (tons). As thecasing material of the heat-resisting ferritic cast steel in the presentinvention, a satisfactory one can be prepared in such a way thatalloying raw materials to be brought into the desired composition aremelted by an electric furnace and then refined by a ladle, and that theresulting raw materials are thereafter cast into a sand mold. The caststeel in which casting defects such as shrinkage holes are involved insmall numbers, can be obtained by sufficiently refining and deoxidizingthe raw materials before the casting.

After the cast steel has been annealed at 1000-1150 (°C.), it isnormalized by heating it to 1000-1100 (°C.) and then quenching it.Subsequently, the resulting steel is tempered twice in the sequence of atemperature range of 550-750 (°C.) and a temperature range of 670-770(°C.). Thus, the steam turbine casing which is usable in the steam at orabove 620 (°C.) can be manufactured. When the annealing and normalizingtemperatures are below 1000 (°C.), carbonitrides cannot be sufficientlyturned into a solid solution, and when they are excessively high, graincoarsening is caused. Besides, the two tempering operations decomposeretained austenite entirely, so that the steel can be rendered thetempered martensitic structure which is homogeneous. Owing to the abovemethod of preparation, the 625-°C. 10⁵ -h creep rupture strength of atleast 10 (kgf/mm²) and the room-temperature absorbed impact energy of atleast 1 (kgf-m) are attained, and the prepared steel can be fabricatedinto the steam turbine casing which is usable in steam at or above 620(°C.).

The casing material in the present invention is set at the Cr equivalentstated before, and the δ ferrite content thereof should preferably be 5(%) or less, and more preferably 0 (%).

Except for the inner casing for the intermediate-pressure steam turbine,which is made of cast steel, the components mentioned before shouldpreferably be made of forged steel.

(3) Others:

(a) The rotor shaft of the low-pressure steam turbine should preferablybe made of low-alloy steel having a fully-tempered bainitic structurewhich contains in terms of weight, 0.2-0.3 (%) of C, at most 0.1 (%) ofSi, at most 0.2 (%) of Mn, 3.2-4.0 (%) of Ni, 1.25-2.25 (%) of Cr,0.1-0.6 (%) of Mo and 0.05-0.25 (%) of V. The low-alloy steel shouldpreferably be manufactured by the same manufacturing method as theferritic steel of the high-pressure and intermediate-pressure rotorshafts, as explained before. Especially, the manufacture shouldpreferably be a superclean (highly pure) one employing raw materialswhich contain at most 0.05 (%) of Si and at most 0.1 (%) of Mn, and inwhich impurities such as P, S, As, Sb and Sn are decreased to the utmostso as to amount to 0.025 (%) or less in total. It is favorable that eachof the P and S contents of the raw materials is at most 0.010 (%), thateach of the Sn and As contents is at most 0.005 (%), and that the Sbcontent is at most 0.001 (%).

(b) Blades for the low-pressure turbine except a final-stage moving one,and nozzles therefor should preferably be made of fully-temperedmartensitic steel which contains 0.05-0.2 (%) of C, 0.1-0.5 (%) of Si,0.2-1.0 (%) of Mn, 10-13 (%) of Cr and 0.04-0.2 (%) of Mo.

(c) Both inner and outer casings for the low-pressure turbine shouldpreferably be made of cast carbon steel which contains 0.2-0.3 (%) of C,0.3-0.7 (%) of Si and at most 1 (%) of Mn.

(d) The casings of a main-steam stop valve and a steam control valve forthe low-pressure turbine should preferably be made of fully-temperedmartensitic steel which contains 0.1-0.2 (%) of C, 0.1-0.4 (%) of Si,0.2-1.0 (%) of Mn, 8.5-10.5 (%) of Cr, 0.3-1.0 (%) of Mo, 1.0-3.0 (%) ofW, 0.1-0.3 (%) of V, 0.03-0.1 (%) of Nb, 0.03-0.08 (%) of N and0.0005-0.003 (%) of B.

(e) A Ti alloy is employed for the final-stage moving blade of thelow-pressure turbine. More specifically, the Ti alloy contains 5-8(weight-%) of Al and 3-6 (weight-%) of V for the length of thefinal-stage moving blade exceeding 40 (inches), and these contents canbe increased with the length. Especially, a high-strength materialcontaining 5.5-6.5 (%) of Al and 3.5-4.5 (1) of V is preferable for thelength of 43 (inches), and a high-strength material containing 4-7 (%)of Al, 4-7 (%) of V and 1-3 (%) of Sn for the length of 46 (inches).

(f) Outer casings for the high-pressure and intermediate pressure steamturbines should preferably be fabricated of cast steel of fully-temperedbainitic structure which contains 0.05-0.2 (%) of C, 0.05-0.5 (%) of Si,0.1-1.0 (%) of Mn, 0.1-0.5 (%) of Ni, 1-2.5 (%) of Cr, 0.5-1.5 (%) of Moand 0.1-0.3 (%) of V, and which favorably contains at least either of0.001-0.01 (%) of B and at most 0.2 (%) of Ti.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional design view of a high-pressure steam turbine madeof ferritic steel according to the present invention.

FIG. 2 is a sectional design view of an intermediate-pressure steamturbine made of ferritic steel according to the present invention.

FIG. 3 is a sectional design view of a low-pressure steam turbineaccording to the present invention.

FIG. 4 is an arrangement diagram of a coal-fired power plant accordingto the present invention.

FIG. 5 is a sectional-view of a rotor shaft for the high-pressure steamturbine according to the present invention.

FIG. 6 is a sectional view of a rotor shaft for theintermediate-pressure steam turbine according to the present invention.

FIG. 7 is a graph showing the creep rupture strengths of rotor shaft andblade materials.

FIG. 8 is a graph showing the relationships between the creep rupturetime periods and Co contents of alloys.

FIG. 9 is a graph showing the relationships between the creep rupturetime periods and B contents of the alloys.

FIG. 10 is a graph showing the relationship between the creep rupturestrengths and W contents of the alloys.

FIG. 11 is a graph showing the creep rupture strengths of casingmaterials.

FIG. 12 is a table (Table 1) exemplifying the specifications of a boilerwhich is operated under specified steam conditions.

FIG. 13 is a table (Table 2) indicating the specifications of a steamturbine which is operated under specified conditions.

FIG. 14 is a table (Table 3) for explaining the alloy contents of steelmaterials which are employed in the present invention.

FIG. 15 is a table (Table 4) for explaining the mechanical propertiesand heat treatment conditions of the steel materials which are listed inTable 3.

FIG. 16 is a table (Table 5) indicating the chemical constituents ofwelding rods which were used in buildup welding.

FIG. 17 is a table (Table 6) indicating those welding rods in Table 5which were used in the respective layers of the buildup welding.

FIG. 18 is a table (Table 7) for explaining the strengths of the rotorshaft and blade materials (in FIG. 7) according to the presentinvention.

FIG. 19 is a table (Table 8) indicating the chemical constituents ofinner casing materials according to the present invention.

FIG. 20 is a table (Table 9) indicating the test results of the innercasing materials (in FIG. 19) according to the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

(Embodiment 1)

Due to sudden rises in the prices of fuel after the Oil crisis, apulverized-coal direct-fired boiler and a steam turbine at steamtemperatures of 600-649 (°C.) have been required in order to increasethermal efficiencies on the basis of enhanced steam conditions. Oneexample of the boiler which is operated under such steam conditions, isindicated in Table 1 of FIG. 12.

Since steam oxidation is attendant upon such a higher-temperatureoperation, 8-10-% Cr steel is employed instead of conventional 2.25-% Crsteel. Besides, since the maximum sulfur content and the maximumchlorine content become 1 (%) and 0.1 (%), respectively, regardinghigh-temperature corrosion ascribable to pulverized-coal direct-firinggas, an austenitic stainless steel pipe functioning as a superheatertube is made of a material which contains 20-25 (%) of Cr and 20-35 (%)of Ni, which contains Al and Ti in very small amounts of at most 0.5(%), and 0.5-3 (%) of Mo, and which more preferably contains at most 0.5(%) of Nb. Pulverized-coal direct firing becomes high-temperatureburning. Accordingly, it is desirable, from the view point of decreasingnitrogen oxides NO_(x), to employ a burner which makes flames of highertemperatures by feeding inner peripheral air and secondary outerperipheral air that form burning flames based on the primary air andpulverized coal, and also reducing flames around the burning flames.

The pulverized-coal fired boiler becomes larger in size as its capacityenlarges. The boiler has a width of 31 (m) and a depth of 16 (m) in theclass of 1050 (MW), and a width of 34 (m) and a depth of 18 (m) in theclass of 1400 (MW).

Table 2 in FIG. 13 indicates the main specifications of a steam turbineplant which has an output of 1050 (MW) and a steam temperature of 625(°C.). In this embodiment, a cross-compound type 4-flow exhaust systemis adopted, and a final-stage blade in each of the low-pressure turbines(LP's) is 43 (inches) long. An HP (high-pressure turbine)--IP(intermediate-pressure turbine) connection has a rotational speed of3600 (r/min), while the two LP's have a rotational speed of 1800(r/min). In a high-temperature portion, components are made of principalmaterials which are listed in the table. The high-pressure portion (HP)undergoes the steam temperature of 625 (°C.) and a pressure of 250(kg/cm²). The intermediate-pressure portion (IP) has its steam heated to625 (°C.) by a reheater (R/H), and is operated under a pressure of170-180 (kg/cm²). Steam enters the low-pressure portions (LP's) at atemperature of 450 (°C.), and it is sent to a condenser at a temperatureof at most 100 (°C.) and in a vacuum of 722 (mmHg).

FIG. 1 is a sectional design view of the high-pressure steam turbine.This high-pressure steam turbine is provided with a high-pressure rotorshaft 23 on which high-pressure moving blades 16 are assembled in ahigh-pressure inner casing 18 and a high-pressure outer casing 19surrounding the inner casing 18. The steam at the high temperature andunder the high pressure as stated before is generated by the boilerexplained before. The generated steam is passed through a main steampipe and then through a main steam inlet 28 defined by a flange andelbow 25, whereupon it is guided to the moving blades 16 of thedouble-flow first stage from a nozzle box 38. The first stage has thedouble-flow construction, and eight other stages are disposed on each ofthe two sides of the high-pressure steam turbine along the rotor shaft23. Fixed blades are respectively provided in correspondence with themoving blades 16. The moving blades 16 are double-tenon type tangentialentry dovetail blades, and the first-stage blade is about 35 (mm) long.The length of the rotor shaft 23 between the centers of bearings 1 and 2is about 5.25 (m), and the smallest diameter part of this rotor shaftcorresponding to the fixed blades has a diameter of about 620 (mm), sothat the ratio of the length to the diameter is about 8.5.

The widths of those parts of the rotor shaft 23 on which the movingblades 16 are assembled are substantially equal at the first stage andfinal stage, and they become smaller toward the downstream side of thesteam stepwise at the five types stages, namely the first stage, secondstage, third-fifth stages, sixth stage and seventh-eighth stages. Theaxial width of the assembled part of the second stage is 0.64 times aswide as that of the assembled part of the final stage.

Those parts of the rotor shaft 23 which correspond to the fixed bladesare smaller in diameter than those parts thereof on which the movingblades 16 are assembled. The axial widths of the parts corresponding tothe fixed blades become smaller stepwise from the width between thesecond-stage moving blade and the third-stage moving blade, to the widthbetween the final-stage moving blade and the penultimate-stage movingblade, the latter width being 0.86 times as wide as the former width.Concretely, the axial widths of the parts corresponding to the fixedblades become smaller at the second-sixth stages and the sixth-ninthstages.

In this embodiment, the blades and nozzles of the first stage are madeof materials indicated in Table 3 of FIG. 14 to be explained later,whereas those of all the other stages are made of 12-% Cr steel whichcontains no W, Co or B. The blade parts of the 0moving blades 16 in thisembodiment are 35-50 (mm) long at the first stage, and become longer atthe respective stages from the second stage toward the final stage.

Especially, the lengths of the blade parts of the second-final stagesare set at 65-210 (mm), depending upon the output of the steam turbine.The number of the stages is 9-12. Herein, the lengths of the blade partsat the respective stages increase at ratios of 1.10-1.15 in terms of thelengths of the downstream-side blade parts adjoining the upstream-sideones, and the ratios gradually enlarge on the downstream side.

As stated above, those parts of the rotor shaft 23 on which the movingblades 16 are assembled are larger in diameter compared with those partsthereof which correspond to the fixed blades. In this regard, the axialwidths of the moving-blade assembled parts become larger with thelengths of the blade parts of the moving blades 16. The ratios of theaforementioned axial widths to the lengths of the blade parts of themoving blades 16 are 0.65-0.95 at the second-final stages, and becomesmaller stepwise from the second stage toward the final stage.

As also stated above, the axial widths of those parts of the rotor shaft23 which correspond to the fixed blades become smaller stepwise from thewidth between the second stage and the third stage, to the width betweenthe final stage and the penultimate stage. The ratios of theaforementioned axial widths to the lengths of the blade parts of themoving blades 16 are 0.7-1.7 at the second-final stages, and becomesmaller stepwise from the upstream-side blade part toward thedownstream-side blade part.

The high-pressure steam turbine shown in FIG. 1 further includes athrust bearing 5, a first shaft packing 10, a second shaft packing 11, ahigh-pressure spacer 14, a front bearing box 26, a journal portion 27, ahigh-pressure steam exhaust port 30, a reheat steam inlet 32, and athrust-bearing wear interrupter 39.

FIG. 2 is a sectional view of the intermediate-pressure steam turbine.This intermediate-pressure steam turbine rotates a generator (G in FIG.13) conjointly with the high-pressure steam turbine, by the use of steamwhich is obtained in such a way that steam exhausted from thehigh-pressure steam turbine is heated again to 625 (°C.) by a reheater(R/H in FIG. 13). Herein, the intermediate-pressure turbine has arotational speed of 3600 (revolutions/min). Likewise to thehigh-pressure turbine, the intermediate-pressure turbine includes anintermediate-pressure inner casing 21 and an outer casing 22. It isprovided with fixed blades in opposition to intermediate-pressure movingblades 17. The moving blades 17 have a double-flow construction of sixstages, and they are disposed in a substantially symmetrical arrangementon both sides of an intermediate-pressure rotor shaft 24 in thelengthwise direction thereof. The distance between the centers ofbearings 3 and 4 in which the rotor shaft 24 is journaled, is about 5.5(m). The moving blade of the first stage has a length of about 92 (mm),and that of the final stage has a length of about 235 (mm). The dovetailof the double-flow construction is in an inverted-chestnut shape. Thatpart of the rotor shaft 24 which corresponds to the fixed bladepreceding the final-stage moving blade 17 has a diameter of about 630(mm), and the ratio of the inter-bearing distance of this rotor shaft tothe aforementioned diameter is about 8.7.

The axial widths of those parts of the rotor shaft 24 of theintermediate-pressure steam turbine of this embodiment on which themoving blades 17 are assembled become larger toward the downstream sideof the steam stepwise at the three sorts of stages of the first stage,the fourth and fifth stages and the final stage. The axial width of theassembled part of the final stage is about 1.4 times as large as that ofthe assembled part of the first stage.

Besides, those parts of the rotor shaft 24 of the intermediate-pressuresteam turbine which correspond to the fixed blades are smaller indiameter than those parts thereof on which the moving blades 17 areassembled. The axial widths of the parts corresponding to the fixedblades become smaller toward the downstream side of the steam stepwiseat the four moving-blade stages of the first stage, second stage, thirdstage and final stage, and the axial width at the final stage is about0.7 time as large as the axial width at the first stage.

In this embodiment, the blades and nozzles of the first stage are madeof the materials indicated in Table 3 of FIG. 14 to be explained later,whereas those of all the other stages are made of the 12-% Cr steelwhich contains no W, Co or B. The blade parts of the moving blades 17 inthis embodiment become longer at the respective stages from the firststage toward the final stage. The lengths of the blade parts of thefirst-final stages are set at 90-350 (mm), depending upon the output ofthe steam turbine. The number of the stages is 6-9. Herein, the lengthsof the blade parts at the respective stages increase at ratios of1.1-1.2 in terms of the lengths of the downstream-side blade partsadjoining the upstream-side ones.

As stated above, those parts of the rotor shaft 24 on which the movingblades 17 are assembled are larger in diameter compared with those partsthereof which correspond to the fixed blades. In this regard, the axialwidths of the moving-blade assembled parts become larger with thelengths of the blade parts of the moving blades 17. The ratios of theaforementioned axial widths to the lengths of the blade parts of themoving blades 17 are 0.5-0.7 at the first-final stages, and becomesmaller stepwise from the first stage toward the final stage.

As also stated above, the axial widths of those parts of the rotor shaft24 which correspond to the fixed blades become smaller stepwise from thewidth between the first stage and the second stage, to the width betweenthe final stage and the penultimate stage. The ratios of theaforementioned axial widths to the lengths of the blade parts of themoving blades 17 are 0.5-1.5, and become smaller stepwise from theupstream-side blade part toward the downstream-side blade part.

The intermediate-pressure steam turbine shown in FIG. 2 further includesshaft packings 12 and 13, an intermediate-pressure spacer 15, a firstinner casing 20 (associated with the second inner casings 21), reheatsteam inlets 29, steam exhaust ports 30, crossover pipes 31, and awarming steam inlet 40.

FIG. 3 is a sectional view of the low-pressure turbine. Two low-pressureturbines are connected in tandem, and they have the same design. Movingblades 41 are provided as eight stages on both sides of a rotor shaft 44in the lengthwise direction thereof, and these moving blades on bothsides are substantially in a bilaterally symmetric arrangement. Besides,fixed blades 42 are disposed in correspondence with the moving blades41. The moving blade 41 of the final stage is 43 (inches) long, and ismade of a Ti-based alloy. The moving blades 41 of all the stages aredouble-tenon type tangential entry dovetail blades. A nozzle box 45 isof double-flow type.

The Ti-based alloy is subjected to age hardening, and it contains 6 (%)of Al and 4 (%) of V in terms of weight. The rotor shafts 44 are made offorged steel of fully-tempered bainitic structure prepared fromsuperclean materials (high purity materials) which consist of 3.75 (%)of Ni, 1.75 (%) of Cr, 0.4 (%) of Mo, 0.15 (%) of V, 0.25 (%) of C, 0.05(%) of Si, 0.10 (%) of Mn, and the balance of Fe. All the moving bladesand the fixed blades except the final-stage ones are made of 12-% Crsteel containing 0.1 (%) of Mo. Cast steel containing 0.25 (%) of C isemployed as the material of the inner and outer casings. The distancebetween the centers of bearings 43 in this embodiment is 7500 (mm).Those parts of the rotor shaft 44 which correspond to the fixed blades42 have a diameter of about 1280 (mm), while those parts thereof onwhich the moving blades 41 are assembled have a diameter of about 2275(mm). The ratio of the inter-bearing distance to the smaller diameter ofthe rotor shaft 44 is about 5.9.

In the low-pressure turbine of this embodiment, the axial widths of themoving-blade assembled parts of the rotor shaft 44 gradually enlarge atthe five sorts of stages of the first-third stages, the fourth stage,the fifth stage, the sixth-seventh stages and the eighth stage. Thewidth of the final stage is about 2.5 times as large as that of thefirst stage.

Besides, those parts of the rotor shaft 44 which correspond to the fixedblades 42 are smaller in diameter than those parts thereof on which themoving blades 41 are assembled. The axial widths of the partscorresponding to the fixed blades 42 become larger toward the downstreamside of the steam gradually at the three sorts of moving-blade stages ofthe first stage, the fifth-sixth stages and the seventh stage, and thewidth at the final stage is about 1.9 times as large as the width at thefirst stage.

The blade parts of the moving blades 41 in this embodiment become longerat the respective stages from the first stage toward the final stage.The lengths of the blade parts of the first-final stages are set at90-1270 (mm), depending upon the output of the steam turbine. The numberof the stages is 8 or 9. Herein, the lengths of the blade parts at therespective stages enlarge at ratios of 1.3-1.6 in terms of the lengthsof the downstream-side blade parts adjoining the upstream-side ones.

As stated above, those parts of the rotor shaft 44 on which the movingblades 41 are assembled are larger in diameter compared with those partsthereof which correspond to the fixed blades 42. In this regard, theaxial widths of the moving-blade assembled parts become larger with thelengths of the blade parts of the moving blades 41. The ratios of theaforementioned axial widths to the lengths of the blade parts of themoving blades 41 are 0.15-0.91 at the first-final stages, and becomesmaller stepwise from the first stage toward the final stage.

Also, the axial widths of those parts of the rotor shaft 44 whichcorrespond to the fixed blades 42 become smaller stepwise from the widthbetween the first stage and the second stage, to the width between thefinal stage and the penultimate stage. The ratios of the aforementionedaxial widths to the lengths of the blade parts of the moving blades 41are 0.25-1.25, and become smaller stepwise from the upstream-side bladepart toward the downstream-side blade part.

Apart from this embodiment, it is also possible to similarly construct alarge-capacity power plant of 1000 (MW) class in which steam inlets to ahigh-pressure steam turbine and an intermediate-pressure steam turbineare at a temperature of 610 (°C.), while steam inlets to twolow-pressure steam turbines are at a temperature of 385 (°C.).

FIG. 4 is a diagram showing the typical plant layout of a coal-firedhigh-temperature high-pressure steam turbine plant.

The high-temperature high-pressure steam turbine plant in thisembodiment is chiefly configured of a coal-fired boiler 51, ahigh-pressure turbine 52, an intermediate-pressure turbine 53,low-pressure turbines 54 and 55, steam condensers 56, condensate pumps57, a low-pressure feed water heater system 58, a deaerator 59, apressuring pump 60, a boiler feed pump 61 and a high-pressure feed waterheater system 63. Herein, ultra-supercritical steam generated by theboiler 51 enters the high-pressure turbine 52 to generate power.

Thereafter, exhaust steam from the high-pressure turbine 52 is reheatedby the boiler 51, and the resulting steam enters theintermediate-pressure turbine 53 to generate power again. Exhaust steamfrom the intermediate-pressure turbine 53 enters the low-pressureturbines 54 and 55 to generate power, and it is thereafter condensed bythe condensers 56. The resulting condensate is sent to the low-pressurefeed water heater system 58 and deaerator 59 by the condensate pumps 57.Feed water deaerated by the deaerator 59 is sent by the pressurizingpump 60 and boiler feed pump 61 to the high-pressure feed water heatersystem 63, in which the water is heated and from which it is returned tothe boiler 51.

Here in the boiler 51, the feed water is turned into high temperatureand high pressure steam by passing through an economizer 64, a vaporizer65 and a superheater 66. Meantime, the combustion gas of the boiler 51having heated the steam comes out of the economizer 64, and itthereafter enters an air heater 67 to heat air. In the illustratedplant, the boiler feed pump 61 is driven by a boiler feed pump drivingturbine which is operated by steam extracted from theintermediate-pressure turbine 53.

In the high-temperature high-pressure steam turbine plant thusconstructed, the temperature of the feed water having emerged from thehigh-pressure feed water heater system 63 is much higher than a feedwater temperature in the prior-art thermal power plant, and hence, thetemperature of the combustion gas having emerged from the economizer 64disposed in the boiler 51 becomes much higher than in the prior-artboiler as an inevitable consequence. Therefore, heat is recovered fromthe exhaust gas of the boiler 51 so as to prevent the gas temperaturefrom lowering.

Numerals 68 in FIG. 4 indicate generators which are respectively joinedto the HP-IP connection and the tandem LP connection.

By the way, apart from this embodiment, it is possible to similarlyconstruct a tandem compound type power plant in which the samehigh-pressure turbine, intermediate-pressure turbine and one or twolow-pressure turbines as described above are joined in tandem so as torotate a single generator for power generation. In the generator whoseoutput power is in the 1050 (MW) class as in this embodiment, a shaft ofhigher strength is employed for the generator. Especially, the generatorshaft should preferably be made of steel of fully-tempered bainiticstructure which contains 0.15-0.30 (%) of C, 0.1-0.3 (%) of Si, at most0.5 (%) of Mn, 3.25-4.5 (%) of Ni, 2.05-3.0 (%) of Cr, 0.25-0.60 (%) ofMo and 0.05-0.20 (%) of V, and which has a room-temperature tensilestrength of 93 (kg/mm²) or above, particularly 100 (kg/mm²) or above,and a 50-% FATT (Fracture Appearance Transition Temperature) of 0 (°C.)or below, particularly -20 (°C.) or below. The steel should preferablybe such that a magnetizing force at 21.2 (kG) is at most 985 (AT/cm),that impurities P, S, Sn, Sb and As contained is at the total amount ofmost 0.025 (%), and that a ratio Ni/Cr is at most 2.0.

FIGS. 5 and 6 are front views showing examples of the high-pressure andintermediate-pressure turbine rotor shafts, respectively. Theexemplified high-pressure turbine shaft has a construction whichconsists of a multistage side and a single-stage side, and in whichblades totaling eight stages are assembled on both sides so as tolaterally center on the first-stage blade of the multistage side. Theexemplified intermediate-pressure turbine shaft has a construction inwhich multistage blades are assembled in a bilaterally symmetricarrangement so as to total six stages on each side and to besubstantially bounded by the laterally central part of this shaft.Although the rotor shaft for each low-pressure turbine is notspecifically exemplified, the rotor shaft of any of the high-pressure,intermediate-pressure and low-pressure turbines is formed with a centerhole, through which the presence of defects is examined by a ultrasonictest, a visual test and a fluorescent penetrant inspection.Incidentally, numerals 27 in each of FIGS. 5 and 6 denote the journalparts of the corresponding rotor shaft.

Table 3 in FIG. 14 indicates the chemical constituents (weight-%) whichwere used for the principal components of the high-pressure turbine,intermediate-pressure turbine and low-pressure turbines in an example ofthis embodiment. In this example, all the high-temperature parts of thehigh-pressure and intermediate-pressure turbines were made of the steelmaterials of ferritic crystalline structure which had a coefficient ofthermal expansion of 12×10⁻⁶ (/°C.), so that no problems ascribable todiscrepancy in the coefficients of thermal expansion occurred.

Regarding each of the rotors of the high-pressure andintermediate-pressure portions, an electrode was prepared in such a waythat the heat-resisting cast steel mentioned in Table 3 was melted in anamount of 30 (tons) by an electric furnace, that the molten steel wassubjected to carbon vacuum deoxidation and then poured into a metalmold, and that the molded steel was forged. Further, the electrode wassubjected to electroslag remelting so as to melt from the upper part ofthe cast steel to the lower part thereof, and the resulting steel wasforged into a rotor shape having a diameter of 1050 (mm) and a length of3700 (mm). The forging was carried out at temperatures of at most 1150(°C.) in order to prevent any forging cracks. Besides, after the forgedsteel was annealed, it was heated to 1050 (°C.) and was subjected towater spray quenching. Subsequently, the resulting steel was temperedtwice at temperatures of 570 (°C.) and 690 (°C.), and it was machinedinto the shape shown in FIG. 5 or FIG. 6. In this example, the upperpart side of the electroslag ingot was used as the first-stage bladeside of the rotor shaft, and the lower part side as used as thefinal-stage blade side.

Regarding the blades and nozzles of the high-pressure portion andintermediate-pressure portion, the heat-resisting steel materials alsomentioned in Table 3 of FIG. 14 were melted by a vacuum arc furnace, andthey were forged and molded into the shapes of blade and nozzle blankseach having a width of 150 (mm), a height of 50 (mm) and a length of1000 (mm). The forging was carried out at temperatures of at most 1150(°C.) in order to prevent any forging cracks. Besides, the forged steelwas heated to 1050 (°C.), subjected to oil quenching and tempered at 690(°C.). Subsequently, the resulting steel was machined into thepredetermined shapes.

Regarding the inner casings of the high-pressure portion andintermediate-pressure portion, the casing of each main-steam stop valveand the casing of each steam control valve, the heat-resisting caststeel materials mentioned in Table 3 were melted by an electric furnaceand then refined by a ladle. The resulting materials were thereafterpoured into sand molds. The cast steel, which did not suffer any castingdefects such as shrinkage holes, could be obtained by sufficientlyrefining and deoxidizing the materials before the pouring. Theweldability of each of the casing materials was evaluated in conformitywith "JIS Z3158". A preheating temperature, an inter-pass temperatureand a post-heating starting temperature were set at 200 (°C.), and apost-heating treatment was conducted at 400 (°C.) for 30 (minutes). Nowelding cracks were noted in either of the materials of the presentinvention, and the weldability was good.

Table 4 shown in FIG. 15 indicates the heat treatment conditions of theferritic steel materials listed in Table 3 of FIG. 14, and themechanical properties of the principal members of the high-temperaturesteam turbines made of the materials as tested by cutting these members.

From results of the tests of the central parts of the rotor shafts, ithas been verified that the special qualities (625-°C. 10⁵ -h strength≧13kgf/mm², and 20-°C. absorbed impact energy≧1.5 kg-m) required of thehigh-pressure and intermediate-pressure turbine rotors can besatisfactorily met. Thus, it has been proved that the steam turbinerotors usable in steam of 620 (°C.) or above can be manufactured.

Besides, as the results of the property tests of the blades, it has beenverified that the special quality (625-°C. 10⁵ -h strength≧15 kgf/mm²)required of the first-stage blades of the high-pressure andintermediate-pressure turbines can be satisfactorily met. Thus, it hasbeen proved that the steam turbine blades usable in steam of 620 (°C.)or above can be manufactured.

Further, as the results of the property tests of the casings, it hasbeen verified that the special qualities (625-°C. 10⁵ -h strength≧10kgf/mm², and 20-°C. absorbed impact energy≧1 kg-m) required of thehigh-pressure and intermediate-pressure turbine casings can besatisfactorily met, and that weld metal materials can be deposited tothe casings. Thus, it has been proved that the steam turbine casingsusable in steam of 620 (°C.) or above can be manufactured.

FIG. 7 is a graph showing the relationships between the 10⁵ -h creeprupture strength and temperature for different rotor shaft materials. Ithas been found that the materials according to the present inventionsatisfy the requirements, which the 10⁵ -h creep rupture strength isequal to or stronger than 13 kg/mm², at 610-640 (°C.). Incidentally, the12Cr rotor material is the prior-art material which contains no B, W orCo.

In an example of this embodiment, bearing characteristics were improvedin such a way that Cr--Mo low-alloy steel was deposited on each journalportion of the rotor shaft by buildup welding. The buildup welding wascarried out as stated below.

Welding rods employed in the buildup welding were shielded-arc oneshaving diameters of 4.0 (mm). Table 5 shown in FIG. 16 indicates thechemical constituents (weight-%) of deposited metals which were formedby the welding operations with the shielded-arc welding rods. Theconstituents of the deposited metals were substantially the same asthose of the corresponding weld metals (which shall be called the"welding rods A-D" below).

The conditions of the buildup welding were a welding current of 170 (A),a voltage of 24 (V) and a rate of 26 (cm/min).

Eight layers were welded onto the surface of the parent metal describedbefore, as the buildup welding by combining the used welding rods A-Dfor the respective layers as listed in Table 6 of FIG. 17. The thicknessof each layer was 3-4 (mm), the total thickness of the eight layersforming each of samples Nos. 1-3 was about 28 (mm), and the surface ofeach sample was ground to about 5 (mm).

As the conditions of execution of the welding operations, a preheatingtemperature, an inter-pass temperature and a stress-relief-annealing(SR) starting temperature were 250-350 (°C.), and the SR was conductedby holding the deposited layers at 630 (°C.) for 36 (hours).

All the samples Nos. 1-3 conformed to the present invention, and thechemical constituents of the fifth layer et seq. in each sample were Cor D mentioned in Table 5 of FIG. 16.

In order to confirm the quality of such a welding zone, buildup weldingwas similarly conducted on a flat member, and the resulting flat memberwas subjected to a side-bend test of 160°. All this time, no cracks werenoted in the welding zone.

Further, when the bearings were subjected to a slide test on the basisof the revolutions of the rotor shafts in the present invention, andnone of them were not adversely affected. The oxidation resistances ofthe bearings were also excellent.

Apart from this embodiment, it is possible to similarly construct atandem type power plant in which the high-pressure turbine,intermediate-pressure turbine and one or two low-pressure turbines arejoined in tandem so as to rotate a single generator at 3600 (r/min).

(Embodiment 2)

Each of a number of alloys, having chemical constituents listed in Table7 of FIG. 18, was cast into an ingot of 10 (kg) by vacuum inductionmelting, and the ingot was forged into a rod of 30 (mm-square).Regarding the rotor shaft of a large-sized steam turbine, each of thealloys was quenched under the conditions of 1050 (°C.)×5 (hours) and 100(°C/h) cooling and was subjected to primary tempering of 570 (°C.)×20(hours) and the secondary tempering of 690 (°C.)×20 (hours), bysimulating the central part of the rotor shaft. On the other hand,regarding the blade of the turbine, each alloy was quenched under thecondition of 1100 (°C.)×1 (hour) and was tempered under the condition of750 (°C.)×1 (hour). Thereafter, the creep rupture tests of such alloyswere executed under the conditions of 625 (°C.) and 30 (kgf/mm²). Theresults obtained are also listed in Table 7 of FIG. 18.

It is seen from Table 7 that the alloys No. 1-No. 9 according to thepresent invention have a much longer creep rupture lifetime than thecomparative alloy No. 10.

Incidentally, the comparative alloy No. 10 does not contain Co unlikethe alloys of the present invention.

FIGS. 8 and 9 are graphs showing those influences of the Co content andB content of the alloys (listed in Table 7 of FIG. 18) which arerespectively exerted on the creep rupture strength.

As shown in FIG. 8, the creep rupture time period of the alloy becomeslonger with increase in the Co content. However, the increase of the Cocontent by a large amount is unfavorable for the reason that the alloyis liable to become brittle when heated at 600-660 (°C.). In order toenhance both the strength and toughness of the alloy, therefore, the Cocontent should preferably be 2-5 (%) for 620-630 (°C.) and 5.5-8 (%) for630-660 (°C.).

As shown in FIG. 9, the strength of the alloy is prone to lower withincrease in the B content. It is understood that the alloy exhibits asuperior strength when the B content is 0.03 (%) or below. The strengthis increased by setting the B content to 0.001-0.01 (%) and the Cocontent to 2-4 (%) in a temperature range of 620-630 (°C.), and byincreasing the B content to 0.01-0.03 (%) and the Co content to 5-7.5(%) on a higher temperature side of 630-660 (°C.).

It has been revealed that the alloy is strengthened more by a lower Ncontent at the temperatures exceeding 600 (°C.) in this embodiment. Thisis also apparent from the fact that sample No. 2 in Table 7 of FIG. 18exhibits a higher strength than sample No. 8 having a higher N content.The N content of the alloy should preferably be 0.01-0.04 (%). Since theconstituent N was hardly contained by the vacuum melting, the parentalloy was doped with the element N.

It is seen from FIG. 7 concerning Embodiment 1 that all the alloysaccording to the present invention as listed in Table 7 exhibit highstrengths. The rotor material indicated in Embodiment 1 corresponds tothe alloy of the sample No. 2 in this embodiment.

As shown in FIG. 9, the sample No. 8 having a low Mn content of 0.09 (%)exhibits a higher strength, subject to equal Co contents. As is alsoapparent from this fact, the Mn content of the alloy should preferablybe set at 0.03-0.20 (%) in order to attain a higher strength.

(Embodiment 3)

Table 8 shown in FIG. 19 indicates chemical constituents (weight-%)which concern the inner casing materials of the present invention. Withthe thick part of a large-sized casing assumed, the ingot of each of thelisted samples was prepared in such a way that each alloy was melted inan amount of 200 (kg) by a high-frequency induction furnace, and thatthe molten alloy was poured into a sand mold having a maximum thicknessof 200 (mm), a width of 380 (mm) and a height of 440 (mm). The samplesNos. 3-7 are materials according to the present invention, whereas thesamples Nos. 1 and 2 are materials of the prior art. The materials ofthe samples Nos. 1 and 2 are Cr--Mo--V cast steel and11Cr--1Mo--V--Nb--N cast steel respectively which are currently used inturbines. After having been annealed by furnace cooling of 1050 (°C.)×8(h), the samples were heat-treated (normalized and tempered) under thefollowing conditions, assuming the thick part of the casing of thelarge-sized steam turbine:

Sample No. 1:

Air cooling of 1050 (°C.)×8 (h)

Air cooling of 710 (°C.)×7 (h)

Air cooling of 710 (°C.)×7 (h)

Samples No. 2-No. 7:

Air cooling of 1050 (°C.)×8 (h)

Air cooling of 710 (°C.)×7 (h)

Air cooling of 710 (°C.)×7 (h)

The weldability of each of the samples was evaluated in conformity with"JIS Z3158". A preheating temperature, an inter-pass temperature and apost-heating starting temperature were set at 150 (°C.), and apost-heating treatment was conducted at 400 (°C.) for 30 (minutes)

Table 9 shown in FIG. 20 indicates the test results of the samples Nos.1-7 listed in Table 8 of FIG. 19, concerning tensile characteristics atroom temperature, V-notch Charpy impact absorption energy at 20 (°C.), a650-°C. 10⁵ -h creep rupture strength, and a welding crack.

The creep rupture strength and the absorbed impact energy of each of thematerials of the present invention (samples Nos. 3, 4, 6 and 7) dopedwith appropriate amounts of B, Mo and W, fully satisfy the specialqualities (625-°C. 10⁵ -h strength≧8 kgf/mm², and 20-°C. absorbed impactenergy≧1 kg-m) required of the high-temperature high-pressure turbinecasing. Especially, the samples Nos. 3, 6 and 7 exhibit high strengthsexceeding 9 (kgf/mm²). Moreover, none of the materials of the presentinvention (except the sample No. 3) suffer from welding cracks and allhave good weldability. As the result of a test concerning therelationship between the B content and the welding crack of the alloy,when the B content exceeded 0.0035 (%), the welding crack appeared. Thealloy of the sample No. 3 was considered to be somewhat cracked.Regarding the influences of the constituent Mo on the mechanicalproperties, the alloy whose Mo content was as high as 1.18 (%) had ahigh creep rupture strength, but it exhibited a small impact energyvalue and could not meet the required toughness. On the other hand, thealloy whose Mo content was 0.11 (%) had a high toughness, but itexhibited a low creep rupture strength and could not meet the requiredstrength.

As the result of the investigation of the influences of the constituentW on the mechanical properties, when the W content exceeds 1.1 (%), thecreep rupture strength becomes remarkably high, but when it exceeds 2(%), the room-temperature absorbed impact energy becomes low.Especially, the Ni/W ratio of the alloy is controlled to 0.25-0.75,thereby obtaining the casing material of the heat-resisting cast steelwhich meets a 625-°C. 10⁵ -h creep rupture strength of at least 9(kgf/mm²) and a room-temperature absorbed impact energy of at least 1(kgf-m) that are required of the high-pressure and intermediate-pressureinner casings, and main-steam stop valve and control valve casings ofthe high-temperature high-pressure turbine under a pressure of at least250 (kgf/cm²) at a temperature of 621 (°C.). Especially, the W contentand the Ni/W ratio are respectively controlled to 1.2-2 (%) and0.25-0.75, thereby obtaining the excellent casing material of theheat-resisting cast steel which meets a 625-°C. 10⁵ -h creep rupturestrength of at least 10 (kgf/mm²) and a room-temperature absorbed impactenergy of at least 2 (kgf-m).

FIG. 10 is a graph showing the relationship between the W content andthe creep rupture strength for the alloys explained above. As indicatedin the figure, when the W content is at least 1.0 (%), the strength isremarkably increased, and a value of at least 8.0 (kg/mm²) is attained,especially for a W content of at least 1.5 (%).

FIG. 11 is a graph showing the relationship between the 10⁵ -hour creeprupture strength and the rupture temperature for the alloys explainedabove. The alloy of the sample No. 7 in the present inventionsatisfactorily meets the required strength at temperatures of, at most,640 (°C.).

In an example, alloying raw materials to be brought into the desiredcomposition of the heat-resisting cast steel in the present inventionwere melted in an amount of 1 (ton) by an electric furnace and thenrefined by a ladle, and the resulting raw materials were thereafterpoured into a sand mold. Thus, the inner casing for the high-pressureportion or intermediate-pressure portion as described in Embodiment 1was obtained.

After having been annealed by furnace cooling of 1050 (°C.)×8 (h), thecast steel stated above was normalized by air-blast quenching of 1050(°C.)×8 (h) and was tempered twice by furnace cooling of 730 (°C.)×8(h). The casing which was manufactured by way of trial and which had afully-tempered martensitic structure, was cut and investigated. As aresult, it was verified that the manufactured casing fully satisfies thespecial qualities (625-°C. 10⁵ -h strength≧9 kgf/mm², and 20-°C.absorbed impact energy≧1 kg-m) required of the casing of thehigh-temperature high-pressure turbine of 250 (atm.) and 625 (°C.), andthat it can be subjected to welding.

(Embodiment 4)

This embodiment sets the steam temperature of a high-pressure steamturbine and an intermediate-pressure steam turbine at 649 (°C.) in placeof 625 (°C.) in Embodiment 1, and the construction and size thereof areobtained by substantially the same design as in Embodiment 1. Differenthere from Embodiment 1 are the rotor shafts, first-stage moving blades,first-stage fixed blades and inner casings of the high-pressure andintermediate-pressure steam turbines that come into direct contact withthe steam of the above temperature. In the materials of these componentsexcept the inner casings, the B content and the Co content arerespectively increased to 0.01-0.03 (%) and 5-7 (%) in the foregoingmaterials listed in Table 7 of FIG. 18. Further, as the material of theinner casings, the W content of the material in Embodiment 1 asindicated in Table 3 of FIG. 14 is increased to 2-3 (%), and Co is added3 (%). In this way, required strengths are fulfilled, and the design inthe prior art can be used very meritoriously. More specifically, in thisembodiment, the design concept in the prior art as it is can be used inthe point that all the structural materials to be exposed to the hightemperature are formed of ferritic steel. By the way, since thesecond-stage moving blades and fixed blades of the high-pressure andintermediate-pressure steam turbines are subject to steam inlettemperatures of about 610 (°C.), the material used for the first stagein Embodiment 1 should preferably be employed for these components.

Further, the steam temperature of low-pressure steam turbines in thisembodiment becomes about 405 (°C.) which is somewhat higher than about380 (°C.) in Embodiment 1. However, since the material in Embodiment 1has a satisfactorily high strength for the rotor shafts themselves ofthe low-pressure steam turbines, the same superclean material (highpurity material) is employed.

Still further, although a turbine configuration in this embodiment is ofthe cross-compound type, the tandem type in which all the turbines aredirectly connected can also be realized at a rotational speed of 3600(r.p.m.).

The present invention thus far described brings forth effects as statedbelow.

According to the present invention, it is possible to obtainheat-resisting martensitic cast steel the creep rupture strength androom-temperature toughness of which are high at 610-660 (°C.).Therefore, all principal members for ultra-supercritical pressureturbines at individual temperatures can be made of heat-resistingferritic steel, the basic designs of prior-art steam turbines can beused as they are, and a thermal power plant of high reliability can bebuilt.

Heretofore, an austenitic alloy has been inevitably used at suchtemperatures. It has therefore been impossible to manufacture anondefective large-sized rotor, from the viewpoint of manufacturalproperties. In contrast, according to heat-resisting ferritic forgedsteel of the present invention, the nondefective large-sized rotor canbe manufactured.

Moreover, the high-temperature steam turbine made entirely of theferritic steel according to the present invention does not use theaustenitic alloy which has a large coefficient of thermal expansion.Therefore, the steam turbine has such advantages as being rapidlystarted with ease and being less susceptible to thermal fatigue damage.

What is claimed is:
 1. A steam-turbine power plant having a high-pressure turbine, an intermediate-pressure turbine and a low-pressure turbine, the improvement comprising:steam inlet of each of the high-pressure and intermediate-pressure turbines leading to moving blades of a first stage included in each of said high-pressure and intermediate-pressure turbines being at a temperature of 610°-660° C.; steam inlet of said low-pressure turbine leading to moving blades of a first stage included in said low-pressure turbine being at a temperature of 380°-475° C.; a rotor shaft, said moving blades, fixed blades and a casing, which are included in each of said high-pressure and intermediate-pressure turbines and which are exposed to the temperature of said steam inlet of each of said high-pressure and intermediate-pressure turbines, being made of high-strength martensitic steel comprising 8-13 weight-% of Cr; and a final-stage blade portion of the moving blades in said low-pressure steam turbine having a length over 1016 mm and made of a Ti-based alloy.
 2. A steam-turbine power plant comprising a high-pressure turbine, an intermediate-pressure turbine and a low-pressure turbine,wherein steam inlet of each of the high-pressure and intermediate-pressure turbines leading to moving blades of a first-stage included in each of said high-pressure and intermediate-pressure turbines is at a temperature of 610°-660° C., steam inlet of said low-pressure turbine leading to moving blades of a first-stage included in each of said low-pressure turbine is at a temperature of 380°-475° C., a rotor shaft of each of said high-pressure and intermediate-pressure turbines and, at least, said first-stage one of said moving blades of each of said high-pressure and intermediate-pressure turbines are made of martensitic steel comprising 8-13% of Cr, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 3. A steam-turbine power plant having a high-pressure turbine and an intermediate-pressure turbine which are joined to each other, and two low-pressure turbines which are connected in tandem, the improvement comprising:steam inlet of each of the high-pressure and intermediate-pressure turbines leading to moving blades of a first stage included in each of said high-pressure and intermediate-pressure turbines being at a temperature of 610°-660° C.; steam inlet of said low-pressure turbine leading to moving blades of a first stage included in said low-pressure turbine being at a temperature of 380°-475° C.; the first-stage moving blade of said high-pressure turbine, and that part of a rotor shaft of said high-pressure turbine on which said first-stage moving blade is assembled being held at metal temperatures which are not, at least, 40° C. lower than the temperature of said steam inlet of said high-pressure turbine leading to said first-stage moving blade; the first-stage moving blade of said intermediate-pressure turbine, and that part of a rotor shaft of said intermediate-pressure turbine on which said first-stage moving blade are assembled being held at metal temperature which is not, at least, 75° C. lower than the temperature of said steam inlet of said intermediate-pressure turbine leading to said first-stage moving blade; and said rotor shaft of each of said high-pressure and intermediate-pressure turbines and, at least, said first-stage of one of said moving blades of each of said high-pressure and intermediate-pressure turbines being made of martensitic steel comprising 9.5-13 weight-% of Cr; and a final-stage blade portion of the moving blades in said low-pressure steam turbine having a length over 1016 mm and made of a Ti-based alloy.
 4. A coal-fired power plant having a coal-fired boiler, steam turbines driven by steam developed by the boiler, and one or two generators driven by the steam turbines and capable of generating an output of at least 1000 MW, the improvement comprising:a high-pressure turbine, an intermediate-pressure turbine which is joined to said high-pressure turbine, and two low-pressure turbines included in said steam turbines; steam inlet of each of the high-pressure and intermediate-pressure turbines leading to moving blades of a first stage included in each of said high-pressure and intermediate-pressure turbines being at a temperature of 610°-660° C.; and steam inlet of said low-pressure turbine leading to moving blades of a first stage included in said low-pressure turbine being at a temperature of 380°-475° C., the steam being heated by a superheater of said boiler to a temperature of at least 3° C. higher than the temperature of said steam inlet of said high-pressure turbine leading to the first-stage moving blade thereof, whereupon the heated steam is caused to flow into said first-stage moving blade of said high-pressure turbine. the steam coming out of said high-pressure turbine being heated by a reheater of said boiler to a temperature of at least 2° C. higher than the temperature of said steam inlet of said intermediate-pressure turbine leading to the first-stage moving blade thereof, whereupon the heated steam is caused to flow into said first-stage moving blade of said intermediate-pressure turbine, the steam coming out of said intermediate-pressure turbine being heated by an economizer of said boiler to a temperature of at least 3° C. higher than the temperature of said steam inlet of said low-pressure turbine leading to the first-stage moving blade thereof, whereupon the heated steam is caused to flow into said first-stage moving blade of said low-pressure turbine, and a final-stage blade portion of the moving blades in said low-pressure steam turbine having a length over 1016 mm and made of a Ti-based alloy.
 5. A low-pressure steam turbine comprising a rotor shaft, moving blades assembled on the rotor shaft, fixed blades for guiding inflow of steam to the moving blades, and an inner casing for holding the fixed blades,wherein said moving blades have a double-flow construction in which at least eight stages are included on each side in a lengthwise direction of said rotor shaft, in a bilaterally symmetric arrangement on both sides, and in which first stage of the arrangement is assembled on a central part of said rotor shaft in the lengthwise direction, said rotor shaft has a distance L of at least 7000 mm between centers of bearings in which it is journaled, and a minimum diameter D of at least 1150 mm at its parts which correspond to said fixed blades, a ratio L/D between the distance L and the diameter D being 5.4-6.3, and is made of Ni--Cr--Mo--V low-alloy steel comprising 1-2.5 wt % of Cr and 3.0-4.5 wt % of Ni, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 6. A low-pressure steam turbine comprising a rotor shaft, moving blades assembled on the rotor shaft, fixed blades for guiding inflow of steam to the moving blades, and an inner casing for holding the fixed blades,wherein said moving blades have a double-flow construction in which at least 8 stages are included on each of two sides in an axial direction of said rotor shaft, in a bilaterally symmetric arrangement on both sides, diameters of the parts of said rotor shaft that correspond to said fixed blades are smaller than the diameters of the parts of said rotor shaft that correspond to the assembled moving blades, widths of the rotor shaft parts corresponding to said fixed blades, in the axial direction of said rotor shaft, are stepwise larger on an upstream side of the steam flow than on a downstream side thereof at, at least, three of said stages; a width between a final stage of said moving blades and a stage thereof directly preceding the final stage is 1.5-2.5 times as large as the width between a first stage and a second stage of said moving blades, widths of the rotor shaft parts corresponding to said assembled moving blades, in said axial direction of said rotor shaft, are stepwise larger on the downstream side of said steam flow than on the upstream side thereof at, at least, 3 of said stages, and an axial width of said final stage of said moving blades is 2-3 times as large as an axial width of said first stage of said moving blades, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 7. A low-pressure steam turbine comprising a rotor shaft, moving blades assembled on the rotor shaft, fixed blades for guiding inflow of steam to the moving blades, and an inner casing for holding the fixed blades,wherein said moving blades have a double-flow construction in which at least eight stages are included on each of two sides in a lengthwise direction of said rotor shaft, in a bilaterally symmetric arrangement on both sides, and blade portions thereof have lengths within a range of 90-1300 mm in a region from an upstream side of the steam flow to a downstream side thereof, diameters of parts of said rotor shaft on which said moving blades are assembled are larger than diameters of parts of said rotor shaft corresponding to the said fixed blades, widths of moving-blade assembling parts of said rotor shaft in an axial direction of said rotor shaft are larger on the downstream side than on the upstream side, and their ratios to the length of said blade portions decrease from said upstream side toward said downstream side within a range of 0.15-1.0, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 8. A low-pressure steam turbine comprising a rotor shaft, moving blades assembled on the rotor shaft, fixed blades for guiding inflow of steam to the moving blades, and an inner casing for holding the fixed blades,wherein said moving blades have a double-flow construction in which at least eight stages are included on each of two sides in a lengthwise direction of said rotor shaft, in a bilaterally symmetric arrangement on both sides, and blade portions thereof have lengths within a range of 90-1300 mm in a region from an upstream side of the steam flow to a downstream side thereof, lengths of said blade portions of the respectively adjacent stages are larger on the downstream side than on the upstream side, and their ratios increase gradually toward said downstream side within a range of 1.2-1.7, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 9. A low-pressure steam turbine comprising a rotor shaft, moving blades assembled on the rotor shaft, fixed blades for guiding inflow of steam to the moving blades, and an inner casing for holding the fixed blades,wherein said moving blades have a double-flow construction in which at least eight stages are included on each of two sides in a lengthwise direction of said rotor shaft, in a bilaterally symmetric arrangement on both sides, and blade portions thereof have lengths within a range of 90-1300 mm in a region from an upstream side of the steam flow to a downstream side thereof, widths of parts of said rotor shaft corresponding to said fixed blades, taken in an axial direction of said rotor shaft, are larger on the downstream side than on the upstream side, and their ratios to the lengths of blade portions of the respectively adjacent moving blades on said downstream side decrease stepwise toward said downstream side within a range of 0.2-1.4, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 10. A low-pressure steam turbine comprising a rotor shaft, moving blades assembled on the rotor shaft, fixed blades for guiding inflow of steam to the moving blades, and an inner casing for holding the fixed blades,wherein steam inlet of said low-pressure turbine leading to a first-stage one of said moving blades is at a temperature of 380°-450° C., said rotor shaft is made of low-steel comprising 0.2-0.3 % of C, at most 0.05% of Si, at most 0.1% of Mn, 3.0-4.5% of Ni, 1.25-2.25% of Cr, 0.07-0.20% of Mo, 0.07-0.2% of V and at least 92.5% of Fe, the percentages being given in terms of weight, and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 11. A steam-turbine power plant according to claim 2 wherein said high-pressure turbine has moving blades arranged to have at least seven stages, and each blade portion of the moving blades has a length within a range of 35-210 mm in a region from an upstream side of the steam flow to a downstream side thereof,said intermediate-pressure turbine has moving blades arranged to have a double-flow construction in which at least six stages are bilateral-symmetrically included on each of two sides, and each blade portion of the moving blades has a length within a range of 100-300 mm in a region from an upstream side of the steam flow to a downstream side thereof, and said low-pressure turbine has moving blades arranged to have a double-flow construction in which at least eight stages are bilateral-symmetrically included on each of two sides, and each blade portion of the moving blades has a length within a range of 90-1300 mm in a region from an upstream side of the steam flow to a downstream side thereof.
 12. A coal-fired power plant comprising a coal-fired boiler, steam turbines driven by steam developed by the boiler, and one or two generators driven by the steam turbines and capable of generating an output of at least 1000 MW,wherein said steam turbines include a high-pressure turbine, an intermediate-pressure turbine joined to said high-pressure turbine, and two low-pressure turbines, steam inlet of each of said high-pressure and intermediate-pressure turbines leading to moving blades of a first-stage included in each of the high-pressure and intermediate-pressure turbines is at a temperature of 610°-660° C., steam inlet said low-pressure turbine leading to moving blades of a first-stage included in each of the low-pressure turbine is at a temperature of 380°-475° C., and said moving blades each have a final-stage including a blade portion having a length over 1016 mm and made of a Ti-based alloy.
 13. A steam-turbine power plant according to claim 2, wherein a rotor shaft and, at least, the first-stage of the moving blades of each of said high-pressure and intermediate-pressure are made of high-strength martensitic steel of fully-tempered martensitic structure which exhibits a 10⁵ -hour creep rupture strength of at least 13 kg/mm² at the rotor shaft and at least 15 kg/mm² at the moving blades at a temperature corresponding to the inflowing steam temperature of the first-stage moving blade, and which comprises 9-13 wt % of Cr.
 14. A steam-turbine power plant according to claim 2, wherein a rotor shaft and, at least, the first-stage of the moving blades of each of said high-pressure and intermediate-pressure are made of high-strength martensitic steel comprising 0.05-0.20% of C, at most 0.15% of Si, 0.03-1.5% of Mn, 9.5-13% of Cr, 0.05-1.0% of Ni, 0.05-0.35% of V, 0.01-0.20% of Nb, 0.01-0.06% of N, 0.05-0.5% of Mo, 1.0-3.5% of W, 2-10% of Co, 0.0005-0.03% of B, and at least 78% of Fe, the percentages being given in terms of weight.
 15. A steam-turbine power plant according to claim 2, wherein a rotor shaft and, at least, the first-stage of the moving blades of each of each of said high-pressure and intermediate-pressure are made of high-strength martensitic steel of fully-tempered martensitic structure exhibiting a 10⁵ -hour creep rupture strength of at least 13 kg /mm² at the rotor shaft and at least 15 kg/mm² at the moving blades at a temperature corresponding to the inflowing steam temperature of the first-stage moving blade, and comprising 9-13wt % of Cr, andsaid Ti-based alloy comprises 5-8% of Al and 3-6% of V, the percentages being given in terms of weight.
 16. A steam-turbine power plant according to claim 2, wherein a rotor shaft and, at least, the first-stage of the moving blades of each of said high-pressure and intermediate-pressure are made of high-strength martensitic steel comprising 0.05-0.20% of C, at most 0.15% of Si, 0.03-1.5% of Mn, 9.5-13% of Cr, 0.05-1.0% of Ni, 0.05-0.35% of V, 0.01-0.20% of Nb, 0.01-0.06% of N, 0.05-0.5% of Mo, 1.0-3.5% of W, 2-10% of Co, 0.0005-0.03% of B, and at least 78% of Fe, the percentages being given in terms of weight, andsaid Ti-based alloy comprising 5-8% of Al and 3-6% of V, the percentages being given in terms of weight. 